Never noticed this before. My x749 4x4ws has the 26 12 12 tires as does the x700 2wd 2ws. I had thought all of these used the 26 12 12 tires and wheels. The front tires have smaller rims on the x734 too, 8" diameter vs. 10" on the x749.
If you are shopping I would recommend going with a model with the 26 12 12 rears and 18 8.50 10 fronts.
Don't know why but maybe clearance issues with the mower decks?