Sam is a decently bulky guy in Supernatural, but Jacob is actually bigger!
Though in Brothers Adopted, he’s still currently shorter than Sam (he will eventually top out over four inches in height, the equivalent of 6′5″), he worked out a lot in school and did boxing before he was cursed. This, combined with the physique inherited from his dad results in him being bulkier than both Sam and Dean.
And Sam is currently only 22, so he’s a lot slimmer than he gets in later seasons of Supernatural, as well.
Brothers Apart 