That isn’t quite correct. The piston doesn’t “get hot” and expand into the sleeve. The upper portion of the sleeve expands to fit the piston, the piston doesn't expand to fill the sleeve. If an ABC engine piston seizes it is because the sleeve hasn't expanded enough.

The high silicon content of the ringless aluminum piston allows the piston to expand at a slightly slower rate and to a lesser extent than the plated brass sleeve surrounding it. (of course non-ABC/ABN/AAC engines are different) The whole design and operating principle of the ringless ABC type engine is such that the upper portion of the sleeve expands at operating temperatures to form the tight yet free running, blow-by free combustion chamber. The pinch is almost gone at operating temperatures. This is why there is a much tighter pinch felt when turning the engine over cold. (doing this should be avoided especially with an engine that hasn’t been broken-in)

These ABC type engines are also called “tapered bore” engines because the cylinder sleeve has a slightly smaller diameter or bore at the top of the cylinder than at the bottom of it. The reason for this is because the upper part of the sleeve which is above the transfer and exhaust ports operates at a much higher temperature than at the lower part of the cylinder. The top of the cylinder therefore has a slightly smaller bore than at the bottom to compensate for this expansion when the engine reaches operating temperatures.

Besides the fact that the piston is unable to expand more than the surrounding sleeve, it is also constantly bathed in a fuel/air mixture on the underside (as well as the areas exposed to the transfer ports) which cools it quite a bit. The piston is unable to attain hotter temperatures than the sleeve. If the piston did get hotter and expanded more than the sleeve then the pinch would increase not decrease when the engines run. There isn’t a circumstance in which the piston could expand only once in a while to seize the engine.

The object of the break-in is to avoid damaging or wearing out the critical and delicate upper cylinder piston/sleeve fit while at the same time stress relieving the parts through heat cycling the engine.