The wattage for cylinder vacuum cleaners was always higher because the machine cleaned by suction alone, whereas the uprights had revolving brushes and didnt need as much power. Add to which, a lot of older uprights (and all the Kirbys) use a 'dirty fan' system which means dust and debris passes through the suction fan, which is not ideal, but it does create a good suction on a very low wattage motor. However, good filtration on this type of machine is almost impossible, hence why almost all upright now have cylinder-style 'clean fan' motors.
In the last few years motor wattage has been increased to compensate for the poor internal design of the vacuum cleaner. This is why the Dyson cleans have -by comparison- low wattage motors, because Dyson cleaners are well designed (if not fragile) to make best use of the motor.
In addition, most bagless cleaners are little more than a tank with a crappy filter, so need lots of suction to off-set the fact that the filter is liable to clog almost immediatly, thus reducing the cleaning power to very little.
Sadly, the public now perceive cleaners with high wattage motors to be the 'best' (i can see that some of you guys do too), whereas the fact is that they are running up the electrcity bills and are overheating themseleves into the process. A Vacuum cleaner of 2400 watts is using the same power as a tumble-dryer, yet no one considers a vacuum cleaner to be expensive to run. When all is said and done, you have to ask why the likes of Numatic make cleaners with large dustbags and motors rated around 1200 watts. Its because it 'works'.