Depends on what you define as "smaller and larger". A 5'10 athlete at 140lbs has more surface area to dissipate heat than a 5'4 athlete at the same mass. But if the taller athlete has the same BMI as the shorter, he or she will do worse in the heat at a given pace (w/kg). If an 180 lb rider and a 140lb rider were doing the same absolute output (w) the larger athlete would have the advantage but that would never happen at a professional level.

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