Romeo is shapeshifting?

Bit of an odd one but Romeo appears to have changed shape. I noticed starting from the beginning of this year that his front legs are set further apart and he had more space under his armpit. His elbows are 'out' more, the only way I can describe it is like a bulldog but not as extreme. His ribs have become more prominant, especially the second. His waist looks tighter. I thought last night his haunches and thighs looked more muscular but thought maybe I was imagining things.
So I weighed him today and he's put on a kg. Now he weighs 24kg. Hes always been a big boned stocky boy but I thought staffies stopped growing at two? He hasn't put on any fat, infact he looks slimmer then before, but being so heavy and also so short legged I worry about his health.
Hes two years and four months old. His diet and exercise regime havn't changed. 24kg doesn't sound like a lot but thats three stone and 11 pounds-on a 14 inch dog that doesn't reach my knee!

Muscle is heavier than fat so it could be that he is just gaining muscle and losing fat, and becoming more toned.

They can put on muscle weight until they are 3. Well really most of their lives if exercised properly. If the two ribs are showing he is in excellent shape

24kg at 14" .....