Ours starting playing up when he found out in t-ball that most of the players looked dumbfounded when asked "what do you do if there are players on 2nd and 3rd and the ball is hit to deep left". Those were his favorite topics.
He played up from that time on. In the beginning it was to play with other kids who understood the game. After that he simply wanted to keep playing with the players he'd been playing with for years.
I too believe that playing up did him a lot of good. I also believe it had a second very important benefit. His size had never kept him from being one of the best players when he was younger than the others, so therefore when he got older his size (or lack thereof) didn't become a huge issue for him.