When we got Elmo, he was 10/11 weeks old. He was 25 pounds. Within two weeks, he was up to 32 pounds. The only reason I know is we couldn't carry him up and down the stairs anymore. He was too heavy to carry! It seems like he was packing on weight very quickly early on. Then, it slowed down.
I don't remember exactly when we switched him to adult food, but it was probably when he was four months old. I remember his weight gain slowed down considerably.
So, I think Apollo is growing just fine. I think there will be some spurts where he grows really fast as a pup.