Why is it that the Δ-Δ transformer needs much higher input power than the Δ-Y transformer?
You are confusing power and voltage in your experimental setup.

Let's assume you use three single phase transformers with a 1:1 turns ratio to build your three phase solutions using 415 V 3 phase as the input.

Star-Star would give 415v on the secondary

Delta-Delta would give 415v on the secondary

Star -delta is a stepdown transformer with secondary output voltage of 415/sqrt(3) = 244 V

Delta-Star is a step up transformer with secondary output voltage of 415*sqrt(3) = 705 V