Hey! I can still throw 30lbs around for an hour just fine! … I just happen to weigh a lot more than that as an adult.

Similar to why kids appear to be made of rubber. Not only are they literally more flexible, but a kid running in to a wall has a LOT less energy than an adult plowing in to it. That energy difference has to come from effort.

To go the same speed, adults quite literally have to use a shitload more energy.