Blackburn Mammoth, lifetime guarantee, pumps up fat MTB tyres in no time, to a high pressure, reliable (I’ve had mine for ages anyway, and if it breaks Blackburn will give me a new one) and it might be slightly bigger, but it doesn’t weigh much (it’s hollow after all) and it just works, really well.
Don’t buy a skinny roadie pump for MTB tyres, because when it’s raining, getting dark and freezing cold, and all your mates are waiting for you to fix your third puncture, you’re gonna wish you had a pump that actually worked on Mountain Bike volume tyres rather than took up as little space in your camelbak as possible (which is full of tubes, butties, jelly babies, cameras, elbow pads and all the other stuff us mountain bikers carry around anyway).