It's called Hollywood fat, the women who are called fat/overweight/obese when at most they're a little fat/chubby. With Japanese people, since most of them don't get super fat, to them, a little pudge equals overweight/obese. This is probably also why most fat characters in anime/manga are drawn with ugly facial features, because the Japanese don't find fatness to be sexy/beautiful and don't want to learn how to draw a fat character as sexy/beautiful. How many anime episodes/manga chapters are there where someone gains a little weight and then they treat it a a massive gain and go crazy trying to lose it?
I also see it as an artist's thing where most artists don't know how to draw fatter bodies so they just add a paunch or have plumpish features and treat a character as though they're fat because of this. I figured since most anime/manga creators are dudes who want their women thin, curvy and leggy, they aren't interested in deviating from this and giving us plumper women. They aren't driven to give us hot fat women because they aren't thin, curvy women.
With women vs. men on this. Women are labeled fat when they're curvier and/or has a big butt or her midsection is a little thicker than most women in the story but she doesn't have a belly per se. For dudes, it's either they have a beer gut or they're thick muscular like a wrestler or American Football player and we say they're fat if they have a paunch. We're taught with men to be strong, you have to have a flat stomach and abs. You can't have a gut and be strong, that's not healthy/attractive we say.
This is probably why most anime fans can have screwed up views of plumpness, since the media they consume solely has thin curvy women, actual fat women are something they rarely see. They just start think that a women with a bit of a belly or who's curvier is somehow an obese blob girl who constantly eats cookies and sweets.