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The force required to compress a pump is determined by the diameter(area) of the piston and the pressure. Bigger pistons require higher force for a given pressure. Pumps for mtb tires have bigger piston diameters to pump more volume which is good for filling the relatively large volume of mtb tires. Road tires have much less volume and run higher pressures, so road pumps usually have smaller diameter pistons which make it easier/possible to reach the higher pressures of road tires.
