It's not uncommon for an anime to look different from the manga that it's based on. Oftentimes, it's about censorship; some things that are shown in the manga simply aren't suitable for daytime television slots. Sometimes, it's about constraints; there are times when the art of anime either doesn't or can't reach the same level of quality as the manga within the allotted time or budget. Alterations like these can still be irksome though, especially to fans who have read the manga and look forward to seeing a faithful adaptation.
A fairly common change between anime and manga is the way that women are drawn. An anime may go out of its way to ensure that female characters have a large chest or an hourglass figure. One Piece, for example, is infamous for giving its women oversized chests and impossibly small waists. Kohei Horikoshi's My Hero Academia seems to be yet another victim of altering women's bodies, especially with its main heroine Ochako Uraraka.
One astute commenter's response to a question on Quora sheds more light on this alteration. The question asks "In the anime My Hero Academia, is Ochaco Uraraka considered a plus-size anime character?" The commenter claims that the anime version of Ochako isn't, but the manga version might be. They provide a side-by-side comparison of a scene from both works depicting Ochako in a cheerleader outfit to illustrate this point. Sure enough, Ochako's manga counterpart has a noticeably wider waistline than her anime counterpart.
Another commenter adds to this by going into what it means to be "plus-size." They define plus-size as "a size of clothing designed for people who are larger than average." They, too, illustrate their definition with examples of "plus-size" women in real life. Compared to these women, the anime Ochako has a relatively "average" build.
Other fans provide images of more girls from the anime and manga to further illustrate the difference between anime and manga builds. Some fans note that Ochako's larger build is shared by her classmates. The consensus seems to be that Horikoshi's idea of the female form is closer to what a real woman would look like. The standard put forth by the anime is not the societal norm they would have audiences believe it to be.
Commenters on the post have varying opinions as to what causes these characters to look the way they do. Some say it’s a style choice while others say that the Japanese (and other countries) don't have the same definitions for the in-between points of fat and skinny that Americans do. There's another theory that manga Ochako might not be eating healthily enough because of her low-income upbringing. However, others say she has more meat on her bones from working out. Others mention that it takes strict dieting and exercise to get Ochako's body, something a Pro-Hero-in-training might do. Whatever the reason for the difference in Ochako's design, there seem to be all sorts of ways to either justify or discredit it.
It should be emphasized that Horikoshi isn't necessarily averse to drawing women with unconventional body types. In fact, he's notable for giving several of the women in MHA a more muscular build than what Shōnen Jump readers are used to. Women like Nana Shimura, Rumi Usagiyama, and Cathleen Bate all defy the convention of women having delicate frames; they instead have well-toned bodies that match their strength.
Horikoshi isn't against drawing overweight characters, either. Looking beyond the many, many examples of people who look different because of their Quirks, there are canonically overweight characters in MHA like Fatgum and Dr. Garaki. If Horikoshi wants to draw definitively plus-sized women and has a place for them in his narrative, he may very well add them.
It's safe to say that Horikoshi and the anime staff have different opinions about the female form. Horikoshi himself seems to aim for more realistic body types, but the anime seems content to provide viewers with a more stereotypical figure. At the very least, Horikoshi seems to know how to depict Ochako believably given her age and background. Whether Ochako and her classmates are plus-size remains up for debate.
