I think so.
Why? Because he is bigger than the average 4 month old, he is quickly outgrowing all of the things that were meant for infants to use. For example, he sleeps so well in his swing. I know, I know, he should be in the crib for naps, but the little man loves his swing time. It allows me to get things done around the house, or play with Lily for a good 1.5-2 hours. For an average sized baby this would be fine, but Jack is almost too big for the swing. The infant car seat isn't too big a deal; however, it means we will need to buy a convertible car seat much sooner than expected. He is in 9 month clothing, and is closing in on 12 month sizes. His head is almost as big as his sister's (and she is 2!).
But he is a little baby.
I think this part is going to be a challenge. When people see a bigger baby, or kid, they assume things about that child. About what they can do. And, therefore, their expectations of the child change. I know Jack is 4 months old. I know what he is capable of, and what he is working on being about to do. When others see him though, when they think he is 6, or 7, or 8 months old, they might think he should be able to do other, harder things. I don't know that is a problem now, since he is only an infant. I do see this becoming problem if he stays in this growth curve as he gets older.
Because some might look at my 4 year old and think he is older. When they do that, they will assume he should listen better, or do more, or be more. I know not everyone will act this way, but I have seen it with other kids. The kids who are just bigger for their age. They are only 2 or 3, and people expect of them what they would expect of a 4 or 5 year old because they look bigger, older.
It is hard for me, now, because I look at him and all his baby-ness. I love his baby-ness. His laid back personality. His willingness to just melt into me when we cuddle. He is a momma's boy, and I am soaking all of that in. But when I hold him, it's like holding a 1 year old (tricky, sometimes, because he cannot be held the way I would hold a one year old...).
I hope that I am wrong.
That size won't matter.