Pistons are normally undersized at the top, near the ring grooves, to leave clearance to accommodate the oil scraped off the cylinder wall. The skirt is usually oval, because it only needs to be a close fit on the two thrust faces, at right angles to the gudgeon pin. By making the skirt smaller than the cylinder diameter directly below the gudgeon pin, there is less friction. More importantly, with careful design the piston's thermal expansion can be made to only vary that diameter, parallel to the gudgeon pin, not the diameter across the thrust faces. Automotive pistons have been oval at the skirt in that way since long before WW2. Round pistons would have to have a lot of clearance across the thrust faces to accommodate thermal expansion, and as a result they would "slap" very noisily on cold start-up. There are round pistons in some crude engines, but it is wheelbarrow engineering.
