On my mower when cold one or two full pulls of the cord is required to start it but once hot it will start easily with a partial pull of the cord.
Perhaps when stopping the mower when hot excess fuel (or oil) is entering the cylinder and trying to compress this may be almost impossible with a pull cord. Maybe the way to check this is to remove the spark plug when the mower is hot to see if it turns over easily.
If the problem is expansion of the pistons etc. I would expect the engine turn-over on the starter or pull cord still to be tight.