I just added the game to the GOG downloader, and the little popout on the "box" in the My Games section of my account said the game weighs in at about 593MB, and for a while that's how it showed up in the downloader. Now it reduced the number to 116.5MB. Did GoG slip in a different version of the game and not update the filesize or something? Is this a bug with the downloader? I'm using v0.9.30

Definitely a glitch, because when I tried to run the "116MB" installer, I got a "Corrupted File" error.

Have you tried downloading it with the browser ?

Yes, and it also downloaded the smaller (read: wrong) sized file.

Looks like the file is messed up at the location you're getting it from. This has happened a few times before and it's one of those glitches that only gog's support can solve. You should contact them.

I did, and I was told to disable any proxy servers (which I don't use) and clean out my Temp folders (which I always do by hand, through Disk Cleanup, and with third party cleaning apps).
I was also told to "ping" the server from whence the file comes while downloading it, but I still got the 116MB file, so something's up.
I told "Peter" (the Tech Support correspondent responding to my case) this, and I'm waiting for a response.

Can't you try downloading it from another machine just to rule out some weird problem with the PC you're using ? I'm still leaning to a problem with the file in the specific location you're getting it from, but anything is possible i guess.
How about other (gog) games ? Do they download ok ?

Well, i'll be surprised if this turns out to be a problem at your end, so i really don't know what else you can try.

The tech support operative helping me asked for a bunch of info like my IP and DNS Server numbers, which leads me to believe that he's going to try and see if an old temp file is stored in a relay or something. But that wouldn't work, because I just got a new ISP this past Monday.

What is really weird is the file changing sizes as you were already downloading it but i guess it's possible you just happened to 'be there' when the file went bad.
I know it sucks to have to wait when this kind of thing happens, but sometimes there's no way around it, and unfortunately this seems to be one of those cases.

What puzzles me the most is why the un-corrupt file is so large to begin with. I had Sonic CD for Windows 95, and that was 600MB for the Full Install, probably because of the FMVs, because the music was Redbook Audio (ran right of the disc) I think. The "Whole Can 'o Worms" package is just a DOS port of the console games, and they're not even complete ports. Levels are missing, the "Normal" difficulty is insane (seriously, I don't remember Fifi taking off 15% health per bite on the Genesis version, or even the Win95 SE), and from what I remember from the demo I got in PC Gamer's demo disc, the game only had MIDI music.
Now, both the SNES and Genesis cartridges for BOTH EWJ 1 & 2 average out at 2MB. How does two 2MB games explode into 600MB?

If you're shocked by that you're gonna love this, my install folder of EWJ 1+2 is 1.3 GB ;)

Well, apparently, the GoG installer mounts a virtual "Whole Can 'o Worms" CD, because the music is the High Quality "real instruments" stuff, as heard in the Sega CD/Windows 95 SE versions. I suppose the music must be in FLAC format, otherwise it wouldn't weigh so much.
BTW, the problem is fixed, and it was on the file host's server's end.