Pudgy!Watson is an invention of Hollywood. Doyle describes Watson when he first appears as "as thin as a lath and as brown as a nut" and later--on the few occasions when he is described--as a well built, muscular sort of guy. He was an athlete in university and played rugby for Blackheath. Though Watson stated he wasn't in the same peak condition he was in when he was a pro athlete, he was still in good enough shape to chase around after Holmes, scaling walls and chasing bad guys. All the Paget drawings show a tall, nicely built man. David Burke in the Granada series probably came really close to the Doyle description. Jude Law is also close, but is a too light on the muscles.