This rule would apply to blocks until they are 1 deep, right? Do you envision no check-time or size rule for blocks that are built on? Or a different much more generous rule?
Even if this rule only applys as long as the difference is one Block:
what is if both branches of the fork have the same deep?
And how could one node know for sure it is on the shorter Branch if it can not check the Blocks of the other Branch because they are to large to be transfered or checked?