Well, I can't tell you what's supposed to be happening but I can tell you what is happening with mine!
In terms of muscle tone, he's getting rounder in general. His neck is looking more...filled in, more solid. I don't know what its called, but I've noticed that that muscle at the base of the neck but on the top, that stretches onto the top middle area of the neck from the wither area is really popping out. Looks like a triangle almost?
His bum is getting rounder, but still has miles to go! His back is smooth, but he naturally uses his back decently, so he's always looked filled in that section of his topline.
He definitely has more energy these days!