Need help

kc

I see a lot of posts where people say baby where’s size 4 diapers but only weigh 18 pounds why is that? I want to know because my LO is 21 pounds and still in size 3 should I move him up to 4? What is the reasoning that they use a size bigger because size 3 are from 16-28 lbs. and size 4 are 22-37 lbs. I am just curious should I move my LO up?