Before ordering a truck of our own and deciding between a C-unit (12-foot box required by FedEx CC) and a D-unit (22-foot box required), we had driven D-units for over a year. Our freight analysis revealed that 7% of our loads were true D loads, either by size or weight or both.
Kindly note that such results will varry not only from carrier to carrier but even from driver to driver with the same carrier. Variables include team v. solo, reefer v. dry van, driver qualifications (security clearances, HAZMAT training and such), driver willingness to touch the freight (v. "no-touch"), and the kinds of runs drivers are willing to take.
For example if you have a reefer you will do more electronics and pharmacutical loads than you would if you had a dry van. Such loads tend to be smaller, lighter, and higher paying. New York City offers art loads if you are flagged to carry them and willing to drive in. Art loads tend to be small and light. If you are willing to do Canada runs, some of those will likely be automotive loads, which can be heavier. If your carrier does more fast freight (expedited general freight) than special-care freight (FedEx White-Glove, Panther Special Services), a larger truck may be for you.
Your carrier of choice should be able to tell you (or at least give a general idea) what their fleet averages are for their larger and smaller straight trucks, and for drivers with various credentials. That's a good question to put to a recruiter if you are talking one.
It's not just about the truck. Carrier choice, driver qualifications, load strategy, and business planning have much to do with the kind of freight you end up hauling and money you make.
