From what I know, a great deal of endurance training (as in long distance/duration cycling) tends to induce the body to release copious amounts of cortisol.

Cortisol is usually called the 'stress hormone' and is released as a response to stress and/or anxiety.

With high enough cortisol levels, again, induced by lots of endurance training, the body can get into a catabolic state where it breaks down muscle proteins for energy.

That I'm guessing, relates to the original poster's mention of muscle atrophy (muscle loss/shrinkage) as opposed to hypertrophy (gain/increase).

For a great match sprinter like Nothstein, it makes sense as upper body strength isn't a help for aerobic/endurance cycling.

Although as with anything, you can only change your body type so much - Nothstein would probably never look like Bradley Wiggins

I'm curious - what motorcycle racer was it who made this comment? I follow motorbike racing a bit myself