He actually used steel cables for his initial prototypes, but steel cables have a problem with rolling fatigue that actually gets worse the larger the diameter...so if a tiny steel cable is not strong enough, you might actually have to change the gear dimensions (and therefore ratios) in order to not fatigue them.

Right. Steel cables have to be oiled internally if flexed frequently. This was a major cause of cable failure in gym exercise machines for decades. Newer exercise machines tend to use synthetic flat straps over pulleys.