My guesses would be:

a) leverage; larger radius = easier to turn
b) ability to reach the wheel while sitting on the cockpit side decks.

Blair,
I agree with Gary on both counts. My 1987 MkI came with a 32" wheel, and upgrading to a 40" (standard on the MkII's) was one of my first priorities for those two reasons.

This winters project .... New wheel 40". Same reasons as above. Also when you are at helm there is less movement of your hands for correcting as you sail.

I upgraded from the stock 32 to a 42 inch wheel as the second thing I did when I got the boat home. Big improvement. I wish I had gone to a 44 but the price was right on the one I got. Its really all about being able to steer from the sides of the boat where you can see the sails, and the extra leverage doesn't hurt either when you want a light touch on the helm.

Jason, I have not done the final measurements but can you get a 44" into a MKI? Even with 2" cushions on the seats? I am in the final stages of my design before I build so I may need to rethink the size.... Bigger can be better....

I haven't measured but my 42" wheel has room for me to get a leg past the sides and more than 2 " underneath.

Caprice is a 1990 model, sail# 1050, Std Rig, WK, and I just went outside to my dock here in sunny Florida to measure my wheel. It is 32 inches with 12 inches clearence at the bottom to the cockpit deck.