adlai

If they were larger they could last longer before replacement. This would be very useful for electric bikes and utility bikes. It seems like they’re the small size they are because disc brakes are originally performance mountain bike components so they’re the minimum size required to stop a mountain bike. But with the increase in electric bikes, larger and less maintenance brakes would be helpful.

Polaris OBark

They got even smaller when the industry transitioned from post-mount to flat-mount.

Then they introduced quad piston for heavier mountain bikes, which has more surface area.

E-bikes in general use normal bike brakes, despite being heavier, so they are probably somewhat under-braked.

Trey83

Size, weight, and expense are all reasons for manufacturers to downsize the caliper and pads. Also there is a bit more complexity in dispersing the compressive load over a larger surface are. The larger the pad the more and larger the pistons will need to be to compress the pads.

Troul 

I have noticed the L03A pad area is not entirely covering the piston surface area with 105 brakes. Have not found a set that uses more of the piston area either.
interesinterestingly enough, the 140mm flat bracket is interchangeable with 160mm if flipped upside down.

Canker

and that is why they make 4 piston brakes.

Ironfish653

Bigger rotors, too. I'd venture to say that fitting a 220mm DH-MTB rotor would provide as noticeable an improvement in braking performance instead of the 140mm that came stock on your hybrid without a significantly-more-expensive 4-piston caliper upgrade.

Pad selection can also play a part; cheap pads may perform acceptably, but not last long; (or they may not work well at all, but last a long time) Conversely; a super-agressive "Race" pad may provide a higher level of performance, but also may have a short operational live, as well as being harder on the rotors.

The trick is to find the balance of cost, performance and lifespan that works best for a given rider.
Generally when you try to find a balance of three factors, it's at the expense of one, so pick the two that are important.

Maelochs

I would assume they are sized to provide sufficient stopping force for the intended applications. If you are using a brake system designed for a lightweight human-powered bike on a small electric motorcycle ......

Exactly. And why should it be otherwise?

Think about it ... MTB brakes are usually Much bigger than road brakes, in terms of swept area. Most road bikes top out at 160 mm while MTBs go to 200 or 220 mm diameter. It is not pad size, it is how much area the pads grip and how the heat is dissipated.

Manufacturers could make pads bigger, could make pistons bigger .... they could drive pumps off the crank and use automotive systems. At some point the engineer has to calculate the expected load in specific uses and design to suit that. Insufficient stopping power, nor excessive weight, really help the situation.

If you find you wear out pads too quickly, maybe you have the wrong brakes or the wrong pads for the application. Those downhill racing pads will grip like crazy but won't last, and if you have a 140-mm rotor on a loaded touring bike or an e-bike, look into bigger rotors (and new brackets.)

Make sure your brakes are not dragging, also. Maybe on an e-bike you don't feel the drag, because the e-assist masks the extra friction. You could be wearing your brakes constantly.

Maybe get a better e-bike, with all components designed to work well in that specific application.

Maybe just accept that if you go fast on your e-bike and stop hard a lot, that is one of the outcomes.

It is like saying that sticky tires with a lot of grip and light, supple casings which are great for racing but wear very quickly, should all be really thick and heavy to last longer. People like soft, sticky tires for the improved performance, and pay the price of shorter lifespan in return for more fun.

Maybe just slow down on your e-bike, or brake less? Or, if you ride for max performance, accept that you are choosing more wear on all the moving parts.

I think the question you are really asking is, "Why don't e-bike manufacturers produce brake systems specifically for e-bikes?"

I'd assume the answer there is, "Most riders do fine with MTB braking systems, and the market is not big enough to justify the cost of designing and building bespoke systems for the tiny handful of riders who desire more."