@JWChibi That's because Japan doesn't have an obesity epidemic in their country. The standards of normal weight versus over weight are different than places like USA where 2/3 the population is over weight and 1/3 of the population is obese. (or in other words, half the over weight people are so overweight they are obese or even morbidly obese) What passes for fat in their (Japanese)perception is someone over weight, while most people in USA don't note a person is fat until they are obese. Also, both fat shaming and skinny shaming is a thing, so most people don't even remark on weight at all.
Then there is the fact this is a not only a manga but a
shoujo manga that tends to beautify the characters and your so called "fat girl" protagonist turns into a beautified mildly chubby girl.