Harmful for the same reason as the 'healthy at any size' campaign, and anti progressive because if they claim health at every size, then they deny there's a huge malnutrition problem in America. It would be solving the problem of food deserts and obesity by just saying 'no no these people are actually healthy' instead of making sure they don't live in a fucking food desert.
healthy at any size means it’s not too late to treat your body with respect. One of the things fatphobia, as ingrained as it is in America, does to your psyche is that it treats you as sub-human and incapable of doing the things you want to do for yourself. Changing your state of mind from “i’m a lazy and fat piece of shit” to “It’s never too late to do right by me, I’m ok”; helped me out tremendously and I actually lost weight in the process.
A lot of fatphobes use “health” as an excuse to treat fat people like shit. It’s wrong. It doesn’t seem them as human, but as bodies that need correcting. And it engenders a lot of problems, my Dumb Bitch of a Dad is fat and he would call my brother and I fat, my brother developed ED and anxiety over his looks. I barely talk to the piece of shit nowadays.