Has someone replaced a caliper piston ?

Replaced my whole dragging caliper but decided to inspect what was the problem with the old one, by removing its piston.
Took brutal force by hundreds of hammer strikes. No rust, no wear signs, no wear nor sizing markings in the cylinder or piston. Just brutally tight.
How smooth is its travel supposed to be ?
Do remanufacturers use óversized pistons to match a rebored-machined cylinder ? How does it work ?

 

The seal is tapered to push the piston back in the bore, it merely rolls out a bit, then back in. It should move smoothly, and If you want to take the piston out, you don't beat it with a hammer, you just step on the brake until the piston pops out. Most of the time, a torn dust shield allows water in and rusts the piston. The rust keeps the piston from retracting. You cant safely oversize the piston in a mass production rebuilding outfit. If you were to mix a stock left piston with a oversize right piston, the car would sharply pull to the left until the caliper on the right filled with enough fluid to touch the rotor, then the car would pull to the right due to greater clamping force from a larger piston/fluid surface area.

 

Thanks.
The caliper was less than a year old, about 3000 miles of granny-style driving.
I did push it outwards with the pedal half inch before disconnecting the hydraulics, with no pads. Thought it would be easier to grab it that way later.
Out, on the bench, there was painfully, had no other way. I had to hammer it out spreading hits around the groove. Strongly suspect it would not had come out with compressed air.

The rubber seal and dust bellows like new, piston shiny, smooth with zero scratches. When finally came out, was impossible to fit by hand with everything clean and without the o-ring seal. Would take hammering to fit it back in.
Obviously wrong. Very wrong.
The cylinder surface appears re-machined to my eye, not mirror finish. The rubber dust cover seal is perfect.

The different diameter explanation, understood.

I will take that with an open port, piston should travel fully by hand force. Please confirm.

I will dismantle another seized caliper I kept from 2 years ago and check if the same symptom. I would dare to hone just to ensure free movement and rotation! as something seems oval. If when 'refurbished' did fit, and by use did rotate, explains it.

 

x1. Also new calipers are 'as cheap as chips' even good quality calipers like TRW. Theoretically you should replace calipers in right and left hand pairs. The piston without the seal should slide freely in and out whatever it's orientation. Often overheating (hi friction pads, soft pads, wheelbearing seized etc) distorts the caliper and bore and the piston seizes in the bore. As Chris says, the biggest enemy is rust and corrosion. When reassembling lightly coat the piston and bore with brake grease (ensure it is specifically brake grease) and that will help.