Here's a bunch of basic info jumbled up quickly.

Certain muscles grow better with high/low repetitions. Cycling power is in the legs, so arms are basically wasted weight for cyclists. If one only cycles, their body will adapt as they are used very little, and legs will grow to support the cycling motions. Also, endurance sports lends itself to leaner athletes due to high energy requirements. Muscle growth basically has to do with low vs high repetitions, and nutrition.