Truck sizes aren't always as advertised, and what seems the largest, isn't always the case. Please refer to post below for actual size comparisons for the U-Haul, Budget, and Penske trucks who are sitting as the largest among their fleet.
*Note the actual square feet of floor space. The less floor space, the less room there is to make an appropriate stack. Lesser stacks means taller stacks.
Here are a few other things to consider:
U-Haul trucks typically have the wheel wells on the floor space. This also takes up a lot of unnecessary space.
U-Haul rentals have other equipment attached to the wall. This takes up space.
U-Haul trucks do have a "Mom's Attic" that we did not include in the measurements as it is only suitable for small/light items.
Penske trucks are rarely more than 2-3 years old, and over 150,000 miles. Their box sizes may differ slightly as they are constantly improving their design.