(Had a request to post this answer in a rebloggable form, so here goes)

Thin shaming is not a thing in the same way that misandry is not a thing. Just as “misandry” isn’t actually concerned with attacks on men, thin shaming isn’t really about body policing of thin bodies, which obviously happens. Rather, the concept is about enforcing thin privilege through establishing a false and destructive false equivillence between what a privileged group experiences and what a disenfranchised group experiences. That’s what people do when they suggest that body policing of a thin body is exactly the same as body policing a fat body. No it isn’t. And by asserting them as exactly the same thing, you aren’t standing up for thin bodies, but advertising a disrespect and disregard for fat bodies and for our real oppression.

Saying misandry isn’t real isn’t saying that some cis-male individuals don’t get targeted by women for abuse. Its saying that equating this with misogyny is hurtful and purposefully counterproductive. Its acknowledging that the purpose of such claims is to negate and disregard the abuse that an oppressed and disenfranchised group is subjected to and to recenter attention onto the needs and problems of those who already enjoy great privilege.

I don’t think body policing is justified, but none of what anyone I’ve seen said even comes close to even being body policing. And even if it did, it would be functionally different than what fat shaming is. That is why thin shaming isn’t a thing. Its not about body policing of thin bodies, but of appropriating the language of oppression to use as a tool of oppression. That’s nothing I’m obligated to acknowledge.