Yeah, I know the question wasn’t really answered… but sometimes you have to go where the funny leads.
But I’ve mentioned it in various places before, werewolves in Zombie Roomie adjust to the “ideal” weight for their frame when they’re wolfed out. It just so happens that Lyn is just about that size when she’s in her human form. If you recall when she turned into the two new forms in the Blaine arc, she got bigger and smaller accordingly.
Jack is chubby as a human so when he turns into the wolfman form, he adjusts down. If he were able to turn into the big bad wolf like Lyn did… he’d get bigger.