I'm not an engineer, but there are MANY more parameters to consider, as I'm sure you are aware.

Just one: If you've seen them run, you can see that the 1.20 has a more than 32% increase in power. Just one factor is the heat-dissipating ability of the 1.2oAX vs the .91FX, with the .91 being a bored-out .61 and therefore having a tendency to overheat unless it is run appropriately rich.

Hence the new .95 AX which is a whole new design with thicker cylinder walls, redesigned fins, and (I assume) porting, etc etc etc