I completely agree with Pete. When you are facing 85mph or a spinner bowling at 50mph, being comfortable at the crease and timing is much more important to most players than bat size. Doesn't take a lot of wood to ramp that speed or to send one rocketing through the covers.
In my personal experience, the only difference between my old sponsorship bats and my off the shelf bats is the overall pressing (Among the three brands I have used) and, in my opinion, this tends to make pro bats that bit bigger.
The "Pro" bats tend to be softer pressed and, generally, do not get knocked in. They are used as "Ready to go, out of the packet" bats, but tend to be smashed to splinters after a season of use and need some serious TLC or binning at that point. However from what I have seen, if you are on TV, you will get the better looking wood, or lower density willow, as beauty and size are the key marketing points in the cricket industry (as well as others!).
The "normal" bats are harder pressed and need more time to open up and play their way in. This tends to elongate the life of the bat and therefore provides less of a risk to the batmaker of returns and the usual issues related to retail sales, while being more cost effective to the consumer.
Ultimately, the "optimum" life of the bat is probably the same, though if I am paying £200-£500 for a bat, you can bet on your life that I want one that lasts longer! If I am not paying... Happy days!
However, I may just be tight!