Didn't read the whole thread, but [MENTION=21724]cyccommute[/MENTION] has hit on the answer - heat transfer through walls is a function of temperature difference on either side of the wall, insulation value of the wall, and surface area where greater surface area allows more heat to pass through. Temperature difference is assumed to be the same when comparing two tents, as is insulation (R) value, so a larger tent has more surface area and will absolutely be harder to keep warm.