Says who? As I mentioned above, is Lehkonen some big hulking bruiser? You've brought up Shaw before, he's about the same size or a little smaller than Lehkonen and Leonard too, and Shaw never struggled to engage physically as an NHLer. You don't need to blow people up to create separation, cut to the middle, or play physically, there are tons of players around 5'11 or 6'0 who are very effective forecheckers who use physical play to create space and then skate into it.
Physicality is part of his game, but Leonard is not being drafted "for" his size or to be a "big" player in the same way as Wood or Anderson or Slaf or whoever else we could care to mention. His projection/upside is primarily about his skill/skating/hands, and the physical element is an additional piece which allows him to unlock more value from his skating, shot, and hands than if he were RHP's size and forced to rely predominantly on elusiveness. He's big enough to get a step or a half-step ahead where he'll outskate defenders, he doesn't need to put a defender on his ass every single play to have success.