AETHERPHON, global theremin family

That is, if a Theremin has great linearity but is overly sensitive
   >on the y-axis, then it is still difficult hit notes precisely.
   
Makes sense to me.  The volume could well be done by a flat plate, you don't really need any resolution.  Long skinny antennas have a problem in that they get droopy, and would tend to add problems.  For a given material, there will come a range of workable lengths.