"They're" great not "Their" great! Jeeze, some people never learn!
Is it the calipers or the brake "system" itself that gives out? I would guess that the calipers hold the heat in fairly well and start to boil the fluid (they are cast iron calipers, right?)
(Note that no matter the disk size the same amount of heat will be generated. The larger disk would give a greater surface area to dissapate the heat.)