My take as a former Fat Girl. All of us Fat Girls have had a relationship like this. You like the guy, and the guy actually likes you too. You spend time together, and sometimes it almost seems to maaayyyybe be about to head to something more -- a little more serious flirting, though it gets laughed off as a joke. Haha! Wouldn't that be so funny? Physical intimacy like hugs, snuggles that last a little long, maybe even a kiss. Ha! See you tomorrow girl!
But he would never, ever date you for REAL, because you are a Fat Girl. What would people say? What would it say about him that he really likes a Fat Girl? So, he keeps hanging out because he really loves her company. She's smart and cool. Keeps her interested with his jokey flirts. And when she gets actual interest from an actual suitor, he gets hurt, mad, scared that he's going to lose the intimate company of this girl he really really likes. He says shitty shit about her new guy. Says shitty shit to her about herself.
But really, he's mostly just mad that he doesn't have the balls to date this Fat Girl that he really likes. And he's so jealous and resentful that someone else WILL.
If you've been there girls lemme hear a Halleujah.