My baby seems big for his age?

Just comparing my baby to others at the same age and he seems so much bigger! At his 4 month visit he was just shy of 17lbs. He’s 21 weeks tomorrow and so heavy! I struggle to carry him and lift him. I am glad he’s gaining weight because I gave birth at 37 weeks and he wasn’t gaining weight from breastfeeding so I had to supplement. He also spits up a lot so i was afraid he wouldn’t gain weight. For reference he’s in size 6-9 month clothes, size 3 pampers and they are starting to get snug.