They're the same. I assume the confusion comes from people measuring the front of the piston where it contacts the pad, which is 36mm on both the PP and STI rear, or because the orange 04-07 STI rear calipers were 36mm pistons. Pistons are 40mm on both 08-17 STI rear and PP rear.
This is a picture of some, just to help illustrate. The dust seal fits into the groove and effectively hides the full piston size.
