Same reason sprinters are faster than weightlifters. The focus on strength and combat makes people more bulky and strong, but less agile and fast.
Sprinters are also quite muscular compared to distance runners. Even among sprinters, compare the musculature of average http://blog.cleveland.com/olympics_impact/2008/08/sprinter-gay.jpg with http://images.salon.com/news/feature/2004/08/24/olympics_04_wariner/story.jpg.
Barbarians focus on pure physique, which gives speed, strength, jumping power, etc. A perfect body type for a barbarian type charcter would be someone like http://bestcelebrityworkouts.com/wp-content/uploads/2010/12/vernon-davis1.png, large and imposing along with ridiculous speed, strength, and leaping ability. Assassins and thieves focus on things like stealth, marksmanship, reflexes, climbing, balance, light fingers, etc. Not purely physical things for the most part.
There's no reason for a thief to be physically superior in any way to a barbarian.