My parents are average size(weight). My brother is slim: 5ft11 and 10.5st. My mother was a slim 8st until pregnancies, cesarians and insulin pushed her weight up to 11st, she eats 3 normal meals a day but due to the above her stomach is massive whilst arms and legs like matchsticks, and no bum!
I blame my weight gain since childhood on an addictive personality:food, alcohol and cigs, thankfully I never got into drugs.

I was normal size until I had my first portion of cheese on toast at 4yo and that must've triggered the addiction because I ate cheese practically every day since so my point is you can be born into a family of normal size people, be fed normal portions but develop an addiction to a certain food/s and battle the weight lifelong, unless you get to grips with the addiction and get it under control, most successfully by going cold turkey (scuse the pun lol) from foods that trigger the response to eat more.
ETA: I just read that back to myself...you'd think I had my addiction sorted, if only! :giggle: