I would not worry about that, you are what you are.
You are the way you need to be, if you need to gain, you do that, but you are OK at every level and every point of it.
I guess it has more to do with stupid and limited people. People that watch too much TV and buy into the brain washing and let their opinions manipulated. People that have the erroneous view that there is just one beauty ideal.
Normal smart and developed people are flexible. Ever been able to chose the size or apperance of a lover / date / husband / wife? Nope, you usually have to take what you get. And normal smart people are flexible that way, they can find many things sexy and attractive, you at 200, you at 300, you at 400: just different flavours of you, with different pros and cons, but always sexy and attractive.
So the people that reject you, well, be glad that you don't have to put up with nitwits and idiots that are hung up on some beauty ideal put in their mind by the media and fashion industry and, well, Disney ;-)
Or people that are half way out of the closet, like fat, but still kinda want a thin girlfriend. Be glad that you will not be wasting time with these.
Yes, you might have a smaller group to chose from, BUT you will actually know that they do like you and you won't be number 5234 and be just somebody. I guess it can also happen when your definition of deep friendship changes and you get kinds bored and frustrated at mindless small talk or superficial chit chat that leads nowhere and is so replacable (they could have it with anybody else, they don't need you for that) and does not define us as humans. If you know that like I do, the group of people gets smaller, but much more high quality and meaningful.
I have that feeling sometimes myself, that I am way too thin for women that like fat. But as a mutual gainer, I know that there will be a woman that likes the process, the "turning a thin person into a fatty". With feeders or mutual gainers that expect me to be really fat already I'm therefore incompatible, but I won't miss much, because what feeders are they anyway that the shy away from "the work", the wonderful transformation and watching the fat grow? Then they are more like FAs and for a feedee like me, I don't want to just be loved for being big, but also for growing, because that is why I love to do, what I am good at.