Straight from the horses mouth:

Power Sports Factory on the rx200hp:

'One of the major differences is the compression. The RX200HP is 180psi verses the average 140-150 of other GY6 engines. The reason is the piston size. We (PSF) use a 58.5mm piston on the RX200HP. Hence the moniker HP = High Compression and the 155cc displacement. The 1.5mm makes all the difference in the world on such a small displacement. Also the engine has a Qianjiang QJUSA designed Oil Cooler, no other GY6 does.'