Originally Posted by
SapInMyBlood
Even pro tour de France riders seem to have relatively small legs considering their power output.
Top GC contenders appear to have small legs because they tend to be ectomorphs (thin body type). Thin riders have an advantage, because their power-to-weight ratio is high.
Cycling is an aerobic sport, not a strength sport. Putting out lots of power doesn't require lots of muscle mass, but muscles that are efficient at taking in lots of oxygen+fuel and converting it to energy.
I have seen a few good climbers who have massive quads. They are rare, but they do exist. What matters is their aerobic fitness.