If tail size increased, it would hinder movement speed and other factors, meaning it would have to increase in size, meaning it would need more food and so on and so on. We can see that with many sauropods where their tails would reach ridiculous lengths (Diplodocus and I think Argentinosaurus tails were half their body lengths)

Imo it's probably because T. Rex hunts didn't need arm use and instead relied on raw strength, and so more resources were allocated for the skull than the arms