It's not muscle bulk up working out, it's more targeting people that were lazy (I was) and want to get back into shape. If you want to bulk up, he does have some workout videos on Youtube that use larger bells (Joe Rogan challenge).
Of course you can use any size bell you want for any workout. I started with a 15, went to a 20 after 2 workouts, used that for a while and now use a 35 for a lot of them. I'm not "big", but I have gotten in much better shape since I started.