braver,

Its mostly wear on the piston rings is that causes engines to lose compression. We all know that the rings wear on the surface touching the cylinder walls but they also wear on the surfaces that touch the ring groves and the ring groves also wear so there is a larger path for air to leak past the rings.

This all contributes to leakage which reduces the compression. Thus there is lower pressure and since temperature in the cylinder is proportional to pressure, the temperataure is lower which leads to harder starting.

P E H