The other reason is the kokyu part and I'll make a quick stab at telling why that part increases your "power" as you get older. For a very simplistic example, picture Tohei standing on one leg with his partner pushing on Tohei's forearm to show that Tohei is difficult to move (we could just say that Tohei has "root" or "is rooted"). What Tohei has done basically (this is a very simple example; it can get much more complex) is rearrange the way his body is handling forces so that the responsibility for the force goes to his back leg. If he is letting his back leg handle the push, then he only needs enough strength in the rest of the body to transmit that responsibility. With a lot of practice, the "transmitting" portions of your body can be weirdly soft, but that's another story and it also involve the ki/qi I'm avoiding.
Mike