Originally Posted by FBinNY

+1

Cables fail strand by strand owing to metal fatigue where the cable is repeatedly bent and straightened at the lever mechanism.

As stated, there is a design limitation, but it's made worse by a change in how cables are made. Engineering guidelines for cable pulley radius depend on the type of cable. Wire rope, that some people call braided, ie. 7x7 or 7x19 is more supple than the single center cables, ie. 1x7 typical of gear and and brake cables on bikes.

Over the years we've seen those types of gear cables used on very small diameter lever pulleys like those on downtube shifters with no problems.

However, as I mentioned, there's been a change. In order to make cable run more smoothly in lined housings modern cables are die drawn, distorting the outer wires and squashing it all into a more cylindrical, smoother shape. This lowest friction but at the same time work hardens the wire making it more brittle and increasing the necessary pulley diameter creating today's problem.

Ironically, some people experiencing excessive cable breakage might do better with less expensive gear wires which are not die drawn, being more supple and forgiving of repeated flexing, and extending the amount of time before metal fatigue sets in.