How'd they do that?
As a customer, you do not have to wait for this machine work; you receive re-sized calipers first, make the exchange on your car, then return the old calipers.
The same techniques for high-end automotive restorations are used to manufacture the 40mm version of the Brembo caliper. The pistons are enlarged with a stainless steel sleeve that is interference press fit (to prevent leakage and from sliding off) and finally centerless ground to produce a smooth, perfectly round outer surface.
The castings are bored out from the OEM 36mm size to a larger diameter. Since replacement and repair parts are only available in even number increments, 38mm and 40mm are the only choices. Beyond 40mm risks the wall thickness becoming too thin, so no larger sizes are available.
Making the hole larger is easy; providing a proper piston seal groove is critical. This groove is not simple as it must create a perfect hydraulic seal under tremendous pressure and also produce a small amount of piston retraction when the brakes are not engaged. The retraction ensures the brake pad does not ride against the rotor, which may overheat the pad or rotor, nor create excessive wear.
The fluid is at the top of the image above and on the left side in the images below.
All images from: Anwana, Cai and Chang, Analysis of Brake Caliper Seal-Groove Design, SAE 2002-01-0927
The larger diameter spreads the same brake fluid pressure over a larger area to increase the clamping force: more braking.