Then why even bother to call thin women skinny bitches?

Because whenever you have a larger woman talking about body-acceptance and encouraging larger women to feel good about themselves, it's an absolute stereotype for people to expect them to overreact against the conventional standards of beauty and start disparaging slimmer women.

You see an identical process with larger girls referring to slim women as "not real" women, women with lots of public hair disparaging women who shave as "looking like little girls" and implying their partners are paedophiles, etc, etc. Hell, the number 1 post on 2XC right now (fuck: it's this very post) is one complaining about women with larger labia reacting against labioplasty and a prevailing standard of beauty that emphasises smaller labia by characterising women with small labia and clitorises as having "childish vaginas". How's that for timing?

You can tell this is an instinctive expectation people commonly have, by the sheer number of people who jumped on the one throwaway ironic joke invoking this stereotype in the whole song, to insist that (despite her own subsequent statements explaining and clarifying the line!), Trainor's doing exactly that.

No, she's invoking the stereotype ironically in order to immediately disparage it, in exactly the same way posters on 2XC will post "what about the menz?" or MRAs will post "misandry doesn't real!" or people post "freeze peach!" in SRS.

All of those examples are understood in context to be ironic, and nobody gives them shit about it.

Try the same thing as a celebrity in a light-hearted pop song, however, and half of Twitter starts stamping on your clit because it's an excuse to turn a light-hearted, body-acceptance-encouraging song into a shitstorm of empowering and satisfying recreational offence at the expense of someone people only conceptualise as a two-dimensional cardboard cutout they can tear to shreds to feel better about themselves, rather than a real person who might actually be trying to make a nuanced point in a medium ill-suited to nuance or qualification. :-/