People are very thin in Japan, in general. But I don't think you're correct in your observation: only the older women are like that in Japanese games, like the +30. Young women in japanese games (and hentai in general) are generally thin, or at least well proportioned. The thing is, I guess many nts games are about married couples and thus I guess the woman is often thick.
Even if honestly, we must not play the same games, I mean I've played a few Sakura games the fmc was always rather thin.
But yeah, for women over 30, I noticed that pattern in hentai. Dunno why.
In the west, it's different, in normie games (non-porn) western devs just enjoy making women as ugly as possible.