I bet Lance builds up muscle in the winter so that he has a convenient source of glucose available as he races.
All 20 of the amino acids, excepting leucine and lysine, can be degraded to TCA cycle intermediates as discussed in the metabolism of amino acids. This allows the carbon skeletons of the amino acids to be converted to those in oxaloacetate and subsequently into pyruvate. The pyruvate thus formed can be utilized by the gluconeogenic pathway. When glycogen stores are depleted, in muscle during exertion and liver during fasting, catabolism of muscle proteins to amino acids contributes the major source of carbon for maintenance of blood glucose levels.
He can't afford to carry fat that he won't use, and that would be metabolized inefficiently by his muscles for energy anyway. The benefit of carrying extra muscle is that he gets to use it as he pedals (some of it), and the additional muscle will hold additional glycogen stores that can be used to keep him going.
Basically, he packs on extra muscle because it gives him more sugar stores in his body. So he can push it harder, longer.
This is the real reason that cyclists should lift weights: so their leg muscles will have more mass (protein and glycogen, a carb) in the rest of their muscles to eat when they're hungry.