Is it normal that i feel uncomfortable in my bfs family ?

Okay, so I'm basically the middle one.

See, my boyfriends the middle child so hes got a younger sibling and an older sibling.

His older sibling is married and has a wife and shes twice my weight.

His younger sibling is a girl and shes like half my size.

Even though I'm the middle one I dont like it.

I'd rather be the larger one or the smaller one. I can't even stop myself from thinking about it.

Because I see like if I was the larger one who is around 250 pounds, I wouldnt feel I could ever get to the smaller ones size so I wouldnt feel as upset, but if I were the smaller one..I'd feel like I could gain weight and be healthy and that Id never be obese because shes like 100 pounds.

Is it normal or should I feel okay? I feel like its the worst body type. I feel obesity or underweight is better than my average size because I feel the skinny one judges me...