I’m not sure if I am interpreting this right, are you asking why feminism is in support of very fat bodies but not very thin bodies? or are you asking why feminism doesn’t speak more about anorexia?
if you’re asking the first question, the answer is that fat pride is just one facet of a broader body positivity movement. while pro-ana movements stipulate that only thin bodies are good bodies, and that all bodies should strive to be thin, body posi movements stipulate that all bodies are good bodies and that nobody should have to strive to alter their body out of self-loathing. in that sense feminism is in support of very thin bodies because there are lots of people who are naturally very thin! feminism does not (well, no, it frequently does, but it should not) police bodies. you don’t shame fat bodies and you don’t shame thin bodies either. there is a difference between attacking the bodies of individuals and attacking systems of oppression that promote thinness as the ideal, and encourage women to take dangerous measures to attain that ideal. you may know this system as the ‘beauty industry’ ahaha. I am not being very articulate here but I just want to make clear that it’s nowhere near as simple as turning it into a thin v fat argument. the simplest way I can put it is that feminism doesn’t idealise bodies by creating a body hierarchy; the capitalist, patriarchal beauty industry does.
now, if you were asking the second question that I put forth back up the top, then I agree that feminism doesn’t talk specifically about anorexia and pro-ana communities a lot. but I do think that everything I just mentioned is best understood as a response to an overarching pro-thin mentality. feminism has been doing a pretty good job of proactively rejecting the kind of mentality that you’d find on a pro-ana site. but more to the point, you have to remember that anorexia is a disorder. it’s not voluntary; it’s not a choice. criticising a sufferer of an ED for perpetuating the thin ideal would be akin to criticising a sufferer of depression for being sad all the time. it’s wrong to criticize someone for being anorexic or bulimic or ed-nos or whatever. obviously. it makes much more sense to criticise fashion, film, tv, advertising, the beauty industry etc etc for setting these ideals in the first place than it does to criticise anyone who has simply internalized the messages they’ve been fed and are living them out. I find the overwhelming presence of thinspiration blogs on tumblr really upsetting and problematic, but I think the general approach of matching every thinspo blog with a body posi blog, a fat posi blog, or any blog that advocates for self esteem and self care instead of self hate is a really effective approach.
I feel like nothing I’m writing today has made any sense :( haha I hope that helped though.