The ass is wrong. I think it goes this way: Since the last decade we have seen the popularization of social media and the raising of personality cult. Media like instagram and such has objectified the feminine form and has been reinforced by the globalization of culture. Sadly this culture is reggaeton and american hip hop, the few things is has to offer is the praise of the most basic and primal urges of the human being. That is ass and titties.
i suspect there's a vague correlation between the artistic representation of anime illustration and the influence of global culture. Japan has been not isolated from the rest of the world but its effect has not been very noticeable (and that's a good thing). However, since I frequent the site for more than a decade I've observed a shift in the illustrations propitiated by mobile apps like Kancolle and such, where the quantity of characters allow for a lot of exploration (and maybe it could be exploitation) in relation of body types. Most of these characters now present exceptional proportions relative to bust sizes and asses but what I observed the most is the 'roundness' of them.
This aesthetic has been standardized along the last few years and decreases the variety that many tastes could appretiate, so now we see balloons as tits and asses and it's a trap in which lesser known artists fall while trying to use it as a platform to be more popular in cases such as these. This ass just looks plastic and a copy of a copy. Much alike the ingent quantity of clonated ethots in IG and such.
The cultural influence over japanese artists and the definition of an ideal body aesthetics in the present could offer interesting topics of research.