Size doesn't matter for a workstation, since it is not a portable unit. People do not care if the workstation is big or not...
HP needs more power, since you can add several HDDs, and its motherboard has two processors Intel Xeon E5, not just one as Mac Pro has. Moreover, HP has several PCIe ports, where you can add several cards, which consume a lot of power: up to 4 video cards need a lot of power.
The 1125 W is the maximum power not used everyday.