I dialogued with a Texas breeder of several breeds of chickens who said he believes that the size of a chicken's comb has less to do with heat tolerance than the weight of the bird. He experiences temperatures as high as 112 and says his big heavier breeds perish more than his smaller lighterweight breeds regardless of their comb sizes. Comb sizes have more to do with whether they get frostbite during colder, damp weather so obviously the larger floppy combs will not do well in colder climates.
I personally have experienced that the fluffier and thicker the undercoat of a normal-sized or dual-purpose breed, the less tolerant they are of the heat - for instance, breeds like Ameraucanas, Easter Eggers, Orps, and Hedemoras fall into that category. Our fluffy Ameraucana (my avatar) pants and finds damp soil during heatwaves to sit in while my smaller Silkies are panting but still busy foraging in the heatwave - the Silkies have a walnut comb and the Ameraucana has a pea comb so a smaller comb makes no difference in a heatwave. My Silkies are only 2.4 lbs and the Ameraucana is 5 lbs - the littler fluffy Silkie hens toodle around while the heavily underdowned, bearded, muffed, larger Ameraucana is immobile in a heatwave - she has more body mass and a lot of thick warm underdown that makes her less heat tolerant. We had Leghorns that did well during heatwaves because they had practically no underdown beneath their hard feathers - they didn't have big fluffy butts or petticoats over their long slender legs to trap body heat. They weren't comfortable in the heat but they still remained active.
Apparently pea, rose, and walnut combs have been designed to keep away frostbite rather than designing large combs as heat regulators as previously believed. On our vet's advice - to avoid frostbite on our Leghorns' combs/wattles we used Vitamin E oil in a bottle from Walmart (way less greasy/staining than Vaseline) which is also a beneficial vitamin for skin. We even applied the oil to their legs/feet/toenails/beaks and let the vitamin absorb overnight and there were no stained feathers on the chickens in the morning. Vaseline greases up the feathers so that dust bath soil clings and stains the feathers for weeks whereas the Vit E oil absorbs overnight and doesn't stain the feathers.
