MoonProwler
If you don't want to listen to the truth spoken by the C, D, and E folks, you won't like my comments either, but for those that want to hear from an 18 year van owner/operator leased to FedEx Custom Critical, here're some of the simple reasons that freight that you perceive to be van, or B, freight is placed on larger trucks.
BTW, I'm neither a company man nor an employee of FedEx Custom Critical. I know the below reasons to be facts based upon my experience, knowledge received from other drivers and tidbits I've learned from forum members on Expeditersonlne, attendees at Expedite Expo, Expedite Workshops and the Expediter Experience at the Mid America Trucking Show.
Customer at either end of the run requires a dock high truck;
Freight might require temperature control not available in a van.
Freight might require special handling like furniture pad wrap, pallet jack, air-ride transport, or inside delivery with two-wheel dolly; none of which are usually carried on a van.
Customer might require a ride-along escort for whom you have no room.
Freight is light weight but too tall, too wide, or too long to fit in the 8.5 feet defined as a van load. (it doesn't matter your van internal dimmensions; a B load definition is no larger than 4' x 4' x 8')
Freight is HAZMAT, like poisonous, for which you may not be qualified to carry in a van.
Freight is a White Glove Division load for which you might not be qualified.
Customer requires immediate pickup and larger trucks are polled closer than any vans.
Run plus deadhead miles exceeds the 750 for solo driver or 1100 for team drivers.
Nearby vans may not have had sufficient rest since last load and the next run would exceed the above totals
Eligible vans might all have refused the load.
Nearby vans might be out of service for personal reasons.
Nearby vans might have been placed out of service for failure to submit required documents such as license renewals or van inspection
Freight is an Air Expedite load for which you might not be qualified if your certification is not current.
Freight is a Government load for which you might not possess the required clearances or qualifications.
Your lease may have been terminated because you complain too much, won't listen to reason, are out of servce and not available very much or have a run offer refusal rate that does not meet the carrier's minimum standards.
Company needs a big truck for a pickup near the destination of a van load so, on infrequent occasions they MIGHT put van freight on the big truck to get it to the next place. I should think it would have to be a pretty important load or customer for this to take place.
You might wish to confirm all or any of the above with your contacts at the company.