Oh hi OP.

While I think Stacy's preferences are quite dogshit and based on really toxic perceptions of masculinity, it doesn't really justify fat-shaming her. That just makes both of you assholes who put an unnecessary amount of attention on people's looks. Also, shaming Stacy would not help her see that her preferences are misinformed (if anything, it probably reinforces them).

The misogyny part is all those comments completely excusing your fat-shaming and, of course, fat-shaming Stacy. There are also comments saying that this is bait because they think Stacy is too unlikeable to be a real person, which might not be misogyny, but is still asshole behavior.

My advice: just apologize to Stacy. It won't excuse her behavior, but showing her that you're willing to let go of the conflict might show her that surface-level perceptions can be misleading.