Guess that blows the metabolism theory, unless it's related to metabolic rate and not dominant metabolic mechanism. Most studies suggest that it's linear relationship to body surface area...

Sweat Rates and Fluid Turnover in Professional Football Players: A Comparison of National Football League Linemen and Backs

But if it's not a linear relationship between surface area and sweat rate, wouldn't too much sweat result in a lower body temperature for the heavy sweaters? Or at least they should have an advantage at lower humidity compared to light sweaters (personally, high heat is trivial to me compared to high dew points). Absent in all the studies I can find is a consideration of relative humidity, which may affect different body types and metabolism's in different ways depending on the conditions.