A friend of mine has two kids, a girl and a boy. Girl was 7.5 lbs at birth, boy 9.5. By the time the boy was 1, he outweighed his older sister of 2 years. He was a solid little chunk. She was a teeny little thing. Both completely healthy. Boy takes after mom, girl after dad, down to her food preferences and ability to eat and eat and eat and still be a teeny little thing. Now that she's 5, kinder marvels at how she's a bottomless pit.
Another friend's kid, born 2 days earlier to other boy seems positively lithe next to chunkier boy. HIS sister's development was identical to his, down to the baldness until 2.
I was teeny and while I looked all properly plump and pink , put on weight very slowly. My brother was all long, tomato red (so funny looking) and always looked skinny, but he ate, oh how he ate. And never was a plump baby.
Basically? It's a crapshoot. As long as they aren't starving, you are doing fine.