The struggle against weight loss is universal to cyclists the world over. In order to ride efficiently and effectively, one should have minimal body fat and maintain a diminutive stature, preferably less than 1.57 meters. Speaking from experience as a formidable bike rider at 1.95 meters, weight definitely has advantages. Plunging down any hillock, tolt, hilligen, sinuous mountain track, coulee, downward facing switchback, or sloping driveway allows gravity to work its magic on the bigger rider. Often I find myself reaching subsonic speeds causing facial disfigurement.
Going downhill with extra weight means passing smaller cyclists with ease and getting a gap effortlessly. It means testing your equipment at its most extreme limits and testing your nerve on that acute edge of control. Weight loss takes away the advantage of gravity and that loss should be a consideration when denying oneself that extra gallon of ice cream or six pack of high alcohol beer in the name of svelteness.
Being lighter can mean that a person can climb the same vertical geographic features with aplomb, alacrity and acuity. It can also mean the difference between coughing up a lung/passing out or briskly pedaling away from the competition at the top. A cyclist must ask her or himself how much of a white knuckle thrill of the downhill hurtle can be sacrificed for a more manageable sortie up a mountain pass. Based on Barry Hamlin's calculations (which have been verified by Johan), for every kilo shed by a cyclist, he/she gives up 10 kmph on the downhill. For that same kilogram the cyclist only gains 9 kmph going up the same hill.
This debate is likely to rage for the next 15 minutes.