Why The change ..?

my baby is 4weeks old today.. and he hasn't been finishing his bottles during each feeding like he use to..

When he was first born we had difficulty getting him to eat but he got the hang of it soon and started drinking 4oz at about 2weeks.He's a big baby; was almost 9lbs and taller than average as well. At 3weeks he went from 4oz to often wanting 5oz..and sometime between week3-4 he stopped eating those 5oz at once.Now he eats about 2-3oz before he falls asleep. I always end up trying to feed him the rest 30min later. Sometimes he won't wake up 😅 or open his mouth to finish his bottle so there's at least 1-2oz being wasted.

Anyone been through this as well...? I'm not worried about it because he still eats a lot, I'm just curious.