Not an expert, but an athlete for many years. Endurance events reduce muscle mass because the body can use muscle for energy when it runs out of other fuels. Muscle growth requires more physical exertion for a shorter interval. Two examples. Long distance runners versus sprinters. Two "running" events, two distinct body styles. Sprinters look stocky in comparison. This is also the reason "cheating" sprinters use steriods, while endurance events use doping and EPO. The other example, bicycle sprinters versus TDF riders. This is why interval training is important. You want more muscle while riding? Climb steeper hills, riding for shorter times. Climbing uses you arms as well as your legs. It is the difference in weight lifting between reps and weight.