Body Weight and Heat Tolerance

Here's a snippet from an article covering that ground.  I've seen it suggested elsewhere that the higher ratio of body surface area (for dispersing generated heat) to total body mass makes lighter runners cool more efficiently (hence more heat tolerant).

http://www.ultrarunning.com/ultra/features/training/heat-training-and-acclima.shtml
Heat Training and Acclimatization 

Lighter runners have a thermal advantage
Exercisers lose heat through three main mechanisms: radiation to the surrounding air, convection - which heats a thin layer of air above the skin, and evaporation (mainly of sweat). Heat is generated as a result of metabolism during exercising by the burning of fuel to power muscle contraction and release. In running, about 25 percent of the fuel metabolized actually powers the athlete forward and 75 percent of burned fuel is cast off as heat. Both the rate of heat production and sweat rate are positively correlated (proportional) to body mass (Marino et al., 2000). The Marino studies also showed that heat storage was strongly correlated with body mass at 93 degrees F and moderately correlated at 80 degrees F. To maintain heat balance (production versus loss) a 100 pound athlete could run a marathon at 5:07 per mile pace while a 165 pound athlete could only run at 7:55 minutes/mile pace. The conclusion reached was that lighter runners have a substantial thermal advantage when running in conditions at which heat dissipation processes have reached their limit...

Individual susceptibility
Individual runners will have different susceptibility to heat, dependent upon a number of factors. Heat susceptibility is higher for athletes having a greater stress response, greater body mass, faster running pace, or when in a dehydrated state. Older athletes, those taking diuretics before exercise, and athletes who are not acclimated to heat are more likely to suffer heat effects. As mentioned above, lighter runners have lower metabolic heat production at a given running speed than heavier runners (Dennis and Noakes, 1999).

"Hotter the heat, Harder the steel"

Thanks for the article link, that was exactly what I was looking for!