Pumps are generally high-pressure or high-volume (Yeah, some take a hack at being both and some are in-between), it sounds like you have a high-pressure one that is a bit slow for the MTB. You should be able to find one that will work nicely on the bigger tires. I carry a blackburn Mtn Air pump with my commuter, it is about 10 inches long and and 1.5 inches or so wide. It works sufficiently well that I haven't desired to replace it.