The shorter neck on the instrument also means you don't have to hold your tuning hand out so far (a similar torque issue) and it also means moving your hand out of first position to play higher notes is a shorter motion.
edit: The viola is of course also thicker, which means maneuvering your tuning hand around it involves twisting your shoulder more. I imagine the unusual shape also improves that situation.
I used to play the viola da gamba (leg viola), which comes in all sizes but you play them kinda like little cellos. They have six strings, frets, and you hold the bow a little differently, but even the small ones are played with it tucked between your knees.
The modern viola is too small to resonate well in its note range, but it's a compromise we make to keep it looking like a violin.
[0]: https://en.wikipedia.org/wiki/Violin_octet [1]: https://www.youtube.com/watch?v=EVVh-kNmRcA
Or just have cellists play them, instead of violinists. It seems Yo Yo Ma has played one without much difficulty!