shdw

Because the systems on larger aircraft are more complex and have the capability to handle various ranges of heating/bleed air to each different surface. I would suspect if the tail will gain ice that much easier/faster than the wings this would be somehow accounted for with the larger more complex systems. It appears as you and KC-10 have explained other systems that they keep the tail on and or activate it multiple times in a cycle, nobody seems to know why.

If, say a 777, had a larger quantity of bleed air per square foot distributed to the tail surfaces than it would confirm, to me, that engineers recognize the tendency for smaller surfaces to acquire ice more readily and compensate for that. Systems with boots are not complex enough to allow for such adjustments, such as, heated surfaces, weeping wings, or bleed air surfaces can through quantity, whether it be electricity, liquid, or air, distribution.

Hope that helps. If not, sorry I give up.