5QM 615 123 has a 55mm diameter piston. I have measured it while i had the caliper out.
And yes, the 5QM 615 123 fits on the 8V0 615 125A/126A carriers, however as I said, the piston is positioned a little offset on the brake pad.
The inner part of the pad has less piston surface coverage than the outer part of the pad.
I believe your 8V0 615 123 caliper is the correct fit for the 8V0 615 125A/126A carriers.
So to clarify, the reason why you have a flawless fit is because you are using the 8V0 615 123 calipers in combination with the 8V0 615 125A/126A carriers.
I need to dump the 5QM 615 123 caliper and get the 8V0 615 123 model.
To give a basic illustration of what I mean in case someone does not understand look look at this: