From another thread...

This got me thinking more generally. Why do bikes always seem to have the same brake front and rear? Not just same style but same size, too, and same lever with same pull. Only with disk brakes do you see rear disks that are a little smaller. But on nearly any other wheeled vehicle I can think of, the front brake is several times larger than the rear, and has a caliper to match. Cars often have vented disks in the front that are a couple sizes larger than non-vented in the rear. On both the motorcycles I've owned, the two front disks were about a foot across and the rear was a saucer-sized afterthought, basically only useful in slippery conditions and starting off uphill. If you were an engineer looking to save ounces from one component to strengthen another, why would you put, essentially, two front brakes on the bike?