As funny as it'd be to have Luka pinned to the ground by his tits, I think making them any larger would only be a temporary type thing. I.e. water weight, swelling from lactation or whatever. Proportions wise he's very much on the razor's edge between appealing and too big/stupid looking. So I'm very much content where they are right now. Canonically he's definitely taking medications to pause their growth, though obviously he started them a little bit too late due to just hiding his chest as they developed instead of dealing with them medically.