Mass to contact area is a major factor, but it's not the only one. Locking your brakes is not the fastest way to stop. What you want is to apply the maximum fr

Mass to contact area is a major factor, but it's not the only one. Locking your brakes is not the fastest way to stop. What you want is to apply the maximum friction to the road, this means your brakes have to be letting the disks travel, and at very high speeds your disks and callipers get very hot, very quickly, at which point they stop supplying nearly as much stopping power, and you might in fact no longer be able to lock your brakes. Anti-lock brakes (ABS) are an improvement, because they mean that if you over-brake (locking your brakes) they release them until the wheel starts rotating again, before re-applying the pressure. They're better than locking your brakes, but they're actually less effective than holding your brakes at the optimal pressure. They're just a lot easier to use than it is to apply that pressure manually (particularly as the optimum pressure changes as your speed does and the heat of your disks and callipers).