Yes, OP, it's quite possible that you are too fat, too thin, too tall, too short, too pretty, too ugly, too poor, too rich, too classy, too common... etc.
But if I had to hazard a guess, I'd say you were just too sensitive, too nice, too easily pushed-aside and too ready to feel you aren't liked or welcomed.
The truth is, a group of friends (or neighbours, or workmates) do not NEED you to be their friend, because they have already got established relationships. That doesn't mean they don't like you or won't ever accept you, but you need to be a little more pushy and don't take things to heart quite so much.
Give it time, keep smiling and being friendly, don't assume you are not wanted and keep inviting them to do things etc.
And you're not too fat to have friends!