I am not using Galaxy, and would rather not download the full installer again and again.

There's a wish for this.
That it's even necessary is a disgrace, though.

Devs are having problems even for the galaxy/steam automated updates, so maybe that is also related in this specific case. There are numerous reports of people having to refresh the integrity of the game to re-download parts of the it to avoid crashes at the start and when loading, so maybe in order to work for a system that can't "refresh" integrity they opted to add the complete installers again.

Anyway, as Gloranor said, there are a lot of other games doing the same as you can see in that wish list. I agree it's annoying having to keep 36GB just for this 2 patches the game had. Let's see what happens with the 3rd patch, but don't expect to be different if the devs don't solve first their updating problems.

I am wondering if they are holding off on making patch installer because they are making updates faster than what it would take to make installers and QA test them?

well... i discovered this thread as i was about to post a question/thread of my own about that matter !

i got the game on day one, release day...
given his poor technical state (strong understatement), you can bet i kept my eyes peeled for any update notification for it on my account library since then
i dont use gog galaxy, so dont bother ever mentioning it to me yet, it's not relevant to my point

so today i receive finally an update notification (drowned into 60 more updates for "internal file structure changes" crap... btw what are those changes and WHY, and why flooding the UPDATE notification system if NOTHING changed except said file structure but not the game version or such ?)
hurray ! hallelujah ! finally a patch for bttletech available in the download section

before that, i saw that the changelog in website's version of my library got populated with info about patches 1.01 and 1.02 for the game, patches we NEVER GOT to download (hey, bug fixing and game updates are now GOG GALAXY only, yeah you know the OPTIONAL game client you will require as mandatory if you ever want your bugged games ever fixed)

i download the patch and see the name for the file mentioning "1.02 to 1.03" so it began to smell fishy here, and without a surprise, i got the red error message that this patch/update was not compatible with my game installation
i look at my base installer/setup files and it's not even named the same way, no 1.0 or 1.01 or whatever
it is version 270b_(20256) ! patch now mention that 1.02 would be 20483 and 1.03 would be 20761 under GOG whimsical naming file convention that is completely different from game version...

so now what are my solutions to get my game patched ?
1) redownload 16gb worth of installer ? can you guess how angry, upset and furious such kind of solution makes me like ? especially with my slow connection ? especially as i kept my eyes peeled for three weeks to watch about any incoming patch that never came so far ?

2) keep playing bugged game (some of the fixes mentionned on 1.03 happened to me) , hopine that someday they will deem us low peasants worthy of getting a DECENT incremential patch that work with release version of any version of the game in fact ?

3) install... sorry... INFECT my main computer with GOG Galaxy and update it ? didn't i tell earlier that this was NOT a valid option, for a so claimed "optional" client ? Considering that whenever you launch it after first install, it starts auto updating every game before even letting you turn the feature off 'how do i know ? that's what happened on the other computer i had to infect with it)

4) update galaxy on the second computer i barely turn on any 3 monthes, update galaxy, upload my current Battletech game folder (the 20256 version) over LAN from the main computer to the other, update the game through the galaxy there (did that for antihero once... antihero is barely 150mb, not 16gb), then reupload the patched Battletech back to the main computer over LAN network again and resume playing with slightly less bugs ?

ok this solution ? it is currently the only "possible" workaround that will take me less time (by only a thin stretch) than redownload the 16 whole gb of the main installer
however, again, i cant stress enough how stupid and upsetting it is !

5) ask GOG to release offline incremential patches for release-to-1.01 and for 1.01-to-1.02, as we could expect as customers ? (damn, you tell me there is a damn community wish for that ? god, it is so sick and sad... i mean: having customers supply a "wish" for GOG/store to ever provide game fixes/patches in a decent way)
i guess the answer is "snow ball in hell, we can wait forever"

(not the first time devs cant make gog patches and forces us to redownload entire multi gb sized base installer for minor changes: titan quest, for example, for minor 3mb worth of UI FIXES !, required us to redownload several gb.. and it did it 4 times in a row before they finally were mercyful enough to us to provide incremential patches instead after some while)

TL;DR: damnit gog ! why cant we have patch 1.01 and 1.02 ? providing patch 1.02-to-1.03 as offline installers is USELESS as we non GALAXY user dont have anything else than base release game version, that is obviously uncompatible with this patch Is it ever a sick joke or complete clumsy missed opportunity or lazy job ?
And we even have to make a community wishlist for that ? i'd rather stop now before getting RUDE, frankly ! it makes me sick

and to tell that i have been playing ONLY this game since its release, and despite its awful bugged unstable mess of technical state... That's right: for the last 3 weeks i had only been playing that game... i guess i had been sinking already over 3 digits hours in it, because it's good gamewise, but damn, the game is a complete bugfest (and i can tell that at least 10% of my play time is waiting at slow-as-hell bugged UI and loading times... from an SSD)

Solution for OP:
1) install Galaxy and bite the bullet
2) endure the rage and keep your game unpatched (god knows how much Battletech needs fixing)
3) get your games on STEAM

(no offense, it's just a bitter joke, frankly, but that sounds like the logical conclusion of what would come from any gog support request ticket about current issue, sadly)

I'm on your side, fellow, and i'm quite upset and angry, frankly ! but then again... until any hypothetical "universal" big patch by the end of may... what was this SICK JOKE here ?
i mean providing NO offline/downloadable patch for the last 2 versions, and now putting only a 1.02-to-1.03 patch to download, that is cumpletely useless for people like us who had nothing else than base release version ?

as since the release, nothing else had been available for download (except, of course, through Galaxy)

i mean: here, it's just a sick joke, a pure trolling taunt attempt, this "1.02-to-1.03" knowing we were NEVER given anything close to 1.02 patch to download prior to that.

it just adds to the whole insults, and treats us like idiots, frankly ! it's the closest thing to a rude middlefinger, afaic.

Update:

I tried (just to verify a side theory) to upload my battletech installation to the second computer which has Galaxy installed to it.

after importing and scanning the folder there, it proposed me the 1.03 update... at the cost of downloading 5gb of data !

the 1.02-to-1.03 patch is merely a 350mb download on its own, and given the changelogs i just cant figure out what of the changes listed so far in the 3 patches would require a whooping change of 20% of the whole game's installed files (game folder is 24gb)

not only it is unconvenient, compared to incremential patches to download separately and manually, but i would suspect the GOG Galaxy doesnt even bother (or is ever able of) injecting/changing any data in a file and would rather download the full sized modified file
(as explained in the wishlist's comment/text, such kind of monolithic approach is lazy and completely insane, not to mention very penalizing for people with slow connections, especially when an incremential installer of a tiny fraction of this size could have achieved the same result)

but hey, not entirely's GOG's fault here, Steam is doing that too

strangely enough though, on every computers of my household that either has galaxy or steam, they all seem to bring down my DSL connection on its knees and clogging it, while only efffectively throttling it at half of it (at the very best)
a strange behavior that no reinstall, clean OS setting, network adaptator's drivers update or anything could ever have solved in the past 3 years, while on each of those computers, browser download and gog downloader download and litteraly any third party's game launchers (like Guild Wars 2 mmo launcher, or whatever) are going full speed of said connection without any problem.

still, i find the whole concept of downloading full installed-size files instead of compressed-installer-sized ones is quite lazy, i fail to see any benefit here

so there it is: unpatched game for now for me, and i'll still have to deal with the already solved/fixed issues for now... and i'll wait for a major milestone update later on to ever consider the bother of straining myself to download ful linstaller again (as i can expect no decent universal patch will be supplied)

In the past, people have had some success opening a ticket and having a support guy send him some missed patches. I suspect that this would not be possible in this case, because I suspect that no 101 and 102 patches were ever created. It might be worth a try, though. Before you decide either to go full Galaxy or to bite the bullet and download the 103 installer.

good thing to know about support possibly supplying missing previous patches... won't help me for this one, but i have a couple of games that will benefit from your advice anyway

afaic, i took option 2) keep playing bugged release version and hope/wait and see if they do a decent all-in-one universal patch later or not.

They don't do all-in-one patches at GOG. Instead, the patches check to see if you are trying to patch the right version.

after straining myself downloading the full 12gb installer... i finally have the 1.03 version of the game and...

AND it is even slower and more unstable than before, and i experience a +30% GPU heating compared to before, and CPU usage skyrockets too

how this can even claim the name of "patch"/bugfix ? when it runs slower, for harsher CPU stress and higher GPU temperature increase ?

It runs a fair bit better for me so at least for some the patch does what it should do.
2700k Just hit 27% usage and 1080ti for some reason don't go higher than 84% now.
I did not run Vsync on earlier patches though my fps never really did get past 60 then either.
So all settings maxed with vsync in 4k and a fairly stable 55-60fps.
Pre patch 100% gpu usage with vsync off but fps at around 40-55.

I have a pretty good cpu and installed the patched game on an nvme ssd and it still takes way longer to load up than it should. I have games with much more than Battletech loading way faster.

Battletech needs some serious optimising work. Reminds of Xcom 2 when that came out, it was pretty bad as well until WOTC when suddenly it was loading super-fast.