I don't think you can narrow down to such a specific reason to be honest as like others have said, beauty is in the eye of the beholder. The same reason thin people get overlooked, people with red hair or smaller than average people. We're all so unique that to worry about something like this isn't too productive for our own sense of health.
If I posted a picture of a model online then actually met someone in person... well there's no trust straight away and you can't build a relationship on dishonesty. Being yourself and portraying the true version of you is the best way.
One thing I would say is that the healthier you look, the healthier you feel and this comes across in your personality. If you're truly worried that being overweight can be a dating factor/becoming a roadblock for potential happiness or for finding someone then little changes can have a big impact. I used to be a big guy so know this from experience and it's insane how much better you feel in yourself:-)