Can anybody here confirm if it's possible to change the weight of your character in games like "Koikatsu Party" and "Honey Select 2"?

Almost pathetic, I know, but they both sound amazing enough to try out.
They are said to have some of the greatest character customisation you can find in basically any game.

If nothing, I’d like to be recommended an alternative.
Preferably one smaller than 5 Gigabytes.

Yes you can, but I don’t think you can find a version under 5 gigabytes. There’s a patchy thing you can install to give you all of the mods everyone uses as standard and a pre-installed thingamajig if you somehow can’t be bothered with that. I don’t know if I can actually give you the link for it because it may count as piracy, I could give you a PM if you want.

So there’s basically no alternative to these two games smaller than 10 Gigs?

Admittedly, it’s been a long time since I played Honey Select, but I remember being very underwhelmed by the weight options, even after downloading a raft of mods for the game.

Yes there’s weight sliders, no you can’t make actually obese characters without modding, and even then they don’t really look that great.
I’m sure there’s probably some compressed repack somewhere, but 5GB might be a bit out of reach, maybe.

Koikatsu and some mods ought to give you something passable. That’s a really hard size limit, though.