To the best of my knowledge the "largest" show is the Tucson show. There is a lot of the "same stuff" but it is great to see it all. If you are going for a specific type of animal, you will have a good selection.
The Phoenix show that was at the fairgrounds last fall was much smaller than Tucson but still had a nice selection of different stuff. There is another one coming up in April (East Valley) that has a good amount of vendors on the list.

In my opinion, it is difficult to draw out of state vendors to the Phoenix area due to the economy. I know that it is tuff all over but here in Phx the spending habits of people in the hobby here are really tight. It takes a lot of effort & time to put a show together and not knowing how successful it will be can keep a lot of people from trying. I would never want to tackle this task!

Jim