On 06/05/2017 18:41, DerbyBorn wrote:

Why don't they make the pads just a bit bigger so that they don't allow a
lip to develop on the periphery of the disc?

Heat?
Would the outer edge of the disk get hottest as it's rotating fastest?
Therefore having extra metal beyond the pad gives a bit of a heat buffer
zone to both dissipate heat as it's generated and give a grater cooling
surface and more even temperature under the pad it's self, I think the
reasons aren't just down to a single "because" but the entire
combination of suggested reasons.