Using rpm is not a correct way to indicate power out put. if you look at a power curve for an engine, what you looking at is max rated power vs rpm. there are many lower powers outputs available below that max line for any given rpm below the max line. A better indicator is fuel flow rate since it rate of energy input into the engine. If you do that, you'll will find that ground engagement implements consume more power per foot of working width than mowers. Of course there are other variables For example tilling sand might require less power than mowing 3 inch saplings with a 4 inch spacing. But in general, ground engaging tools with the same working width will require more power than mowers.