Good question to a certain extent. I was faster when I was 180lbs than now that I am 166. Something tells me having big legs is more important than weighing 120lbs until you get to that pro level and you need to optimize your power to weight ratio. My nutritionist told me that cyclist tend to have more fat % relative to runners and triathletes. Something along the lines of 3-4% for ironmen and 5-6% for pro cyclist. I am shooting for that 6% but dang it's hard!