I don't think they try to make them unappealing. It's not the same context as for normi western games. Japanese creators usually don't care about realism (unless it's part of their vision), no matter the field.
The thing is, even if (almost) nobody like whales, the point in which the woman is considered too thick (/fat?) is kinda subjective. Don't get me wrong, I usually prefer women that are relatively thin too, but depending on the woman I'm not against (for example) big boobs either. Not gigantic bazoungas, just big boobs. So I guess the threshold for it being "too much" is just faster than usual for you.
I did a test and I put the swinging tag on vndb.
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Let's consider the first 7 japanese games for example (ignore the other ones, just the japanese/2d games). I only see that green haired woman in the fourth game as being too fat (the belly), and she doesn't seem to be the main female character anyway.
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Apart from that the thickest fmc is probably the 5th game, but honestly I find her ok
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Maybe for you that one is too much ? I think most people would find her rather hot. It's subjective.
The 7th game is even a loli rofl.
The bottom line is, I don't think japanese devs care about political correctness or realism. Once again, I've seen what you're talking about, more in hentai in general than in games though. But I just think some people just like ridiculously big boobs. Maybe because japanese women naturally tend to have small boobs, some men over there are even more obsessed with it ?
