It's a hilarious twist of fate, I swear. I've had this happen with a lot of fat women that I first met in-person.
All throughout high school, only rail-thin girls liked me, and same with my early time in college.
I did finally meet my first serious girlfriend in college who was a little over 200lbs. She just mentally... broke one day, and I'm fairly certain she started cheating on me with any man who gave her the time of day. I always put my all into being the best partner possible for her, so I was genuinely so confused as to why it ended so poorly.
She eventually reached out years later to apologize, and blamed the fact that she was never comfortable being with me; since I was the most conventionally attractive person she had dated, she always felt like she was being judged, and said she always felt like an ogre next to me in public. She even hated our body contrast, it made her self-confidence plummet... so she was actively trying to sabotage the relationship instead of addressing her own body image issues and paranoia.
I've had that same projection/explanation given to me by other fat women I fancied IRL; they thought I was almost taking the piss by romantically pursuing them.
That said, I've never once had a problem with a women I first met on the internet first. Places like Feabie, Fantasy Feeder, and Tumblr (a true FA mecca, RIP) meant that there was zero doubt about my intent, I was here to meet the big behemoth bitches. Lol
It sucks and it shouldn't be this way, but many fat women who actively hate that they're fat will self-sabotage or resign themselves to a certain quality of partner to avoid getting hurt. Maybe, in a lot of these cases, they believe you're too good for them and choose to reject you before you have the chance to do the same.
That said, maybe try meeting a fat lady online in a safe space where they can know that without a doubt, you're looking for what they bring to the table? That's been my only solution.