One thing to remember, hence why towing the motor size does not seem to matter. It takes a certain amount of "calories/btu's etc" to move a given item. so no matter the motor or type, that energy useage is the same. BUT, the same occurs when empty, but some motors like the larger ones, will use more fuel just because they can not shut down as much as a smaller one.
In some instances, where one is not towing a high %, say less than 10% of the time, it might in reality be, just as well to use a smaller motor, know you will get the same mpg as the larger motor, BUT, when commuting, a V6 like the GM 4.3, can get upwards of 22-24 vs the 5.3 or 6.2 V8's will be lucky to get 17-18mpg. As you probably start approaching the 20-40% range, then a larger motor starts to make sense from the actual power perspective and speed up hills to a degree.
Marty
