I think you mean WIDTH. By my measurements of a few samples, disc brake pads are about 14mm wide whereas rim brake pads are about 8mm.
But then, if you're focused only on the pad, there's also LENGTH. Again, from a few samples I measure about 29mm for disc pads and rim pads ranging from 57 to 79mm. By my math that makes the pad surface AREA of rim brakes 10 to 50% larger than that of discs.
I would then suggest that the brake surface area of the disc or rim is at least as important as the pad's surface area. Here's some math on that:
A few notes on these calculations:
- For brake surface diameter on rims I used the 4mm less the the BSD (based on measuring a few rims)
- The 8mm rim brake pad width was based on measuring several samples (WTB Speedmaster, Kool-Stop MTB, Kool-Stop ThinLine)
- The 14mm disc brake pad width was based on measuring pads for a Shimano M970 XTR brake.
So: Rim brakes have a slightly larger pad working on a brake surface roughly DOUBLE that of discs. Now I'm not saying that any of this makes them better or definitively answers the OP's question... but wanted to correct your point about contact area.