Truth be told, the part that the small drive train puts the most extra stress on is the chain. Obviously, we don't exert as much stress as other types of riding, so it's not as much of an issue... but that's why the BMX racers, who are even bigger weight whores than we are, still run the "normal" 44-16 type of gearing; the weight loss of using smaller sprockets and such isn't enough to offset the risk of having a chain snap at the gate, or cranking full-speed along the track.