GSDs don't fill out till all the frame growing is pretty much done. You'll have a lanky dog until the bones are done growing and then they muscle up, in general.
As to activity levels, each dog is different. Dunno if you have kids but kids kinda go through the sullen act when they hit growth spurts. Why not dogs? If it's worrying, have some blood work done and make sure all is right.
Never cook bacon while naked.