I understand that the piston diameter is larger on the six caliper, but externally, they appear to be very similar if not identical.

My question is why can't a rear four caliper be bored to the same piston diameter as the six? Not enough meat on the bone?

Mind you, I understand that if it could be done, PMB probably would not have gone through the trouble to create the GT caliper. Just being curious here.

It's not just the piston, the pads are also slightly bigger.

There is also a difference in the casting

The 4s have a couple of extra ribs

Piston - You would need 2.5mm off the edge of the dust boot flange. There's just not enough meat there to make that happen.

Pad Width - Not only the cavity but the pin spacing would need to be expanded.

All sorts of issues...