It used to be that a compression ratio of 10:1 was the threshold where premium fuel was required. Modern motors, with their oversquare bore/stroke ratios (big bores and short strokes) and highly efficient combustion chambers with 4 valves per cylinder rather than two, require less octane than that old standard would indicate. The more telling figure is one of developed compression, known as Brake Mean Effective Pressure, or BMEP. Bore, stroke, rod length, lift and duration of the cams as well as their timing and overlap all work together to develop high BMEP figures. Higher rpm motors develop their power higher up in the range and the octane needed to do that safely rises with revs. If you never redline your motor, you can get by with less octane. The higher you wind it, the higher the BMEP results and the higher the octane required to avoid detonation. An old, small-bore, long-stroke motor would need 110 octane to preclude detonation with 12:1 compression ratio. Today's flat track Harleys, even though their bore/stroke ratios are not as severely undersquare as in the past, need a good 125-130 octane to cope with their 15:1 compression ratios.
It may be a pig, but it's a pig with a rocket in it's ass!