What size of diapers

My son is 4 months and he's wearing size 3 in diapers. I feel like they are already getting tight on him again, because he has some rolls🤦🏻‍♀️😂 but I think he's going to fast? I mean my niece is a year old and she's size 5 and she's a big heavy baby and I wonder if maybe it's just me or if it's normal to be different? He wears huggies, pampers brand it doesn't matter. But I seriously think this dude is going so fast in sizes😳

This is my 4 month old❤️ wearing size 3