In the early years of GOG many customers (including me) would save the installers to a DVD. And this download size was a perfect fit for DVD - burning.
Hence the reason why some folks started to create custom DVD covers for the GOG games (and the community is still active in the forums).

I suspect most users don't use DVD backup now. But personally, until someone invents a new, bigger and more affordable laser support, I hope that GOG still keeps the 4GB standard.

Not long ago i tried merging and redividing files, not too unlike how you might get say .rar.part1 .rar.part2 or 7z.000 .001 .002, etc. If they were divided from a singular archive like a tar.xz or something then you could just decompress and recompress or merge/split to whatever size you deem appropriate.

Unfortunately that failed. Merging multiple files it complained the files didn't match right, or was missing files. So unfortunately i dis like that part of it. I'd almost prefer if i could resize to perfectly fit DVD discs rather than having say 400Mb leftover, or better split to fit on any sized media like CD's (if it came to that) though the lack of ability to optimize images and the like is another annoyance.

If you are really keen, you could combine GOG's EXE and BIN files into one giant ZIP, 7-ZIP or RAR file. You could even make it a self-extractor one. No doubt you could also do something with MSI, and have the EXE file autorun after extraction.

Hadn't considered that. Though the problem with that is when you go to install it, you use a lot more unnecessary space. Having it execute the installer only hides the crux of the problem that you have to extract it twice to get it to work.

Though not too unlike the deice utility for Doom (Doom disk installer used deice.com or deice.exe, which specified files and sizes, once all the files were on the local machine it ran a SFX which was effectively 'installed' for the demo or whole game).