Tom,
It is hard for me to comment about any relationship between high heat and the size of the plant. A larger plant would likely survive but I suspect you'll still get some leaf burn. I suspect in your area it is not just the heat but the extremely low humidity. We get pretty hot down here in Louisiana but the humidity is always relatively high. Even when it breaks 100, as it did back in June, my naners did fine as long as I watered them. I would speculate your plants can't translocate enough water to the leaf tips & edges so you get some die back from the tips.
Some mulch around the base would help keep the ground cooler and you might consider a misting device when the heat really spikes up. The water police may give you a little trouble.
Good luck and keep us posted on how they do next summer when it warms up again. You might be able to pen an article on how to grow bananas in the desert. Rhett