Cycling does not eat away at muscle mass.

However, upper body muscle mass (except for the little you need to rock the bike on an out of the saddle climb) is just a liability especially in hill climbing. Competition favors cyclists who are as light as possible and anything that is not helping has to go.

Some people said that Lance Armstrong's bout with cancer actually HELPED him as a cyclist. Armstrong had been a triathelete and as such had much more upper body mass then he needed. But he could never get rid of it until he had chemotherapy which pretty much gets rid of everything. So when he recovered and went back into training, he got his cycling muscles back and his aerobic power but never put on the upper body mass. The result was that he was 10-15 lbs lighter which made him a force on hill climbs.

Now bicycling is an aerobic sport. High level aerobic activity requires sugar (glycogen). Once the body has ripped through the stored glycogen, it can only get sugar by ripping up muscle mass and converting it into sugar. If you do a lot of high intensity aerobic exercise and do not eat enough carbos, you could easily end up catabolizing muscle. But that is a matter of the balance of exercise and diet then anything else.

Another thing, is that sports take a fair amount of time. I suppose one could be a decent body builder and a decent cyclist but very few people are. The sports are quite different. Body building does not require anywhere near the aerobic capacity cycling does and a cyclist has no real need to lug around all that upper body weight. So why bother?