It seems that in the 125-160ish range two things make a big difference. The number of valves and water or air cooling. With water cooling the compression ratio can be raised to produce more power without hurting long term reliability. The alternative is a multi-valve head which gives better breathing at higher rpms. A good case in point is the 163cc 2valve Kymco Like 200i which makes 11.3 bhp while the new 4valve Like 150i produces 13.7bhp. Some of the cheaper scooters from mainland China only produce 8-9bhp from 150ccs.