Lance Armstrong doesn't have the body-type for being "huge". I have the same body-type as him. I layed carpet for a year and did heavy lifting the whole time. My muscles got much stronger, on the order of being able to easily lift 100 pounds more, but I only gained about 5 pounds and I didn't grow anywhere. Btw, I am 6ft and 145pounds.
Another reason I think you don't see Armstrong with huge legs is that he opts for a higher pedalling cadence (how fast you pedal, in RPMS) rather than hammering the pedals (lower pedalling speed at a harder, stronger gear). This means his work out is more aerobic, relying on his cardiovascular system to move smaller loads (lower gears) faster. So he isn't pressing 75 pounds each pedal push, rather he is pressing more like 30 pounds (just arbitrary numbers) so hes more of an endurance racer than a pedal masher.
Its like the difference between a power-lifter who lifts 400 pounds three times, and a normal person (for lack of a better word) who lifts 40 pounds 30 times. The power-lifter will be big and bulky but the normal person will be well-defined and able to go for longer periods of time than the power lifter.
Make any sense?