>>718950293
For decades, fan artists and doujin artists had been pushing the boundaries of drawing bodies. It wasn't mainstream but there was a niche market for plump girls. And every few years or so, an anime would attempt to add one plump girl in to test the waters. The character usually didn't go anywhere because of lack of support or just being a secondary/background character. But they tried every once in a while.

Then out of nowhere, a few anime with plump girls got popular around the years 2008-2010. Even though they were still the secondary girls, they gained a lot of attention and got even more popular than the main girl. So, following this interest, many shows started adding one plump girl to the cast.

Most of the games use anime as inspiration. So they just followed the trend. And it's gone from one token plump girl per game to 2-3. And then a couple games where most of the women are thick.