Inline brake levers

Gear cables are thinner than brake cables and the wires a braided differently. The outers are also different. No doubt there's a good reason why the difference but I don't know what it is.

Difference in outer cable is braided (gear) v coiled (brake) - generally, braided brake outer is available, normally reserved for bmx type use though where it has it's own advantages. As far as I know the different types of outer are used because coiled has a bit more give and so provides better brake modulation/less lock ups and braided is nice and stiff so snappy/crisp/precise gear shifts.

Inners are a different size, gear cable being smaller. It has less force/load to carry and if a gear cable breaks, so what. If a brake cable brakes, might cause acrash.