How important is it to have a large surface area on disc brakes? I've seen some brake upgrades that advertise better stopping power with bigger discs than standard but the contact patch with the pads is smaller than the original pads. How does that work? :/

For example, I've just put some bigger brakes on my Mondeo, 30mm bigger infact but garage phoned me up saying the pads are smaller than the ones I had on originally.