"There is a "symbiotic" reason for strings to be thicker for lower tunings. A thicker string provides more
surface area and therefore more raw sound (presence) as needed for a listener to hear tones as the
same "volume" (even thought they are not). A thicker string also povides the necessary physics to
compensate for the different tension that would be required to apply to strings to achieve different
notes... usually more tension for a higher note... yet a thinner string requires less tension to achieve
that note... so the tension across strings/bridges are (theoretically) equal. As for reality...well."..

Dear musick: with fear and trembling consciousness of my ignorance of music, may I observe that it is not only the diameter of the low pitched strings that is important. The lower pitched strings are usually wound with fine metal wire to increase their mass per unit length. The diameter is only one factor in determing pitch.