I think they are just all different. Wally was by far the biggest dog in his litter (he looked like he was a month older than his brothers) but he grew very slowly and all the other dogs surpassed him in weight. Now at 1 year he is fairly tall and weighs 52lbs but his head hasn't really filled out yet. He looks pretty lanky for a Bulldog. This is his most recent picture. He's probably about 10.5 months in it. You can see that his head is not as big as other Bulldogs his age, but is getting bigger every day.
Lets be honest though, I really wrote all this as an excuse to post my new favorite picture of Wally.
View attachment 14975