From what I've read, nutrient partitioning between muscle and fat is optimal around 10-12% bodyfat. Apparently, 20% bodyfat is the upper limit for the optimally anabolic zone. Not to say you can't gain while fatter, just that when lean the body seems to direct more nutrients towards muscle.
Also there's a study demonstrating recovery of testosterone levels when obese folks shed the fat. Fat produces aromatase which turns testosterone into estrogen. Better T levels mean better training.
All this agrees with BtB's advice.