How is my son still in NB sizes 🤔

My son was 9.4 lbs at birth and 19 in. At his 2 week check up he was 9.3 and 21 inches, and that was 3 weeks ago and I am thinking he is near 10 lbs if not over now. Yet 0-3 still just hangs off him and NB fits perfect. He's wearing size 1 diapers that even seem to be getting snug. How is this even possible? He's 6 weeks. I have like 6 NB outfits and 100 0-3 because we thought he wouldn't even fit in newborn. I want him to stop having to wear the same few outfits lol the next size up wardrobe is much cuter.