To add to John's explanation, the expansion tank is there to act as a surge volume, and it only has the small hose going out from the bottom. The loop flow of coolant doesn't pass through the tank. There are only three ways to transfer heat: radiation, convection, and conduction. This setup primarily allows for convection through the coolant, and it can't pass much heat through what is basically a tube of static water 3/8" thick or so.

Add to that the heat loss to ambient air from the tank (again mostly convection from moving air against the tank wall) and you'll find it's hard for the engine to transfer its heat to the exp tank.

That's why the coolant in the radiator will burn your fingers and the coolant in the exp tank won't.

someguyfromMaryland