The cut outs are valve pockets, pretty sure it's to protect the pistons from contact. Compression ratio can be achieve by a shallower piston so if they're exactly the same pistons apart from the CR and the rods are the same length, then yeah the shallower one will have lower compression.
Also oversized does not create compression that's just the diameter of the piston i.e. OEM piston size is 81mm and oversized is anything more than that.
Also oversized does not create compression that's just the diameter of the piston i.e. OEM piston size is 81mm and oversized is anything more than that.