What if you wanted to do something silly with your new, shiny, 315hp/400 ft-lbs truck? like tow? Or even just run some aggressive 33's and and RTT and stuff. Thats where the 18 gallons sucks.
I honestly think Ford does it to keep the Ranger from stepping on the F150's toes. If they made it too capable(like a mid 20's gallon tank and a 6' bed), then suddenly there is a ton of overlap into the 3.3 and 2.7L F150's and we can't have that. There is no reason they couldnt have fit a larger tank in a 130" wheelbase leaf sprung truck when Toyota is able to fit a 23 gallon tank in a 109" wheelbase 4Runner/GX with a linked rear axle.
What if I wanted to go explore the Marianas Trench with my new (used), shiny (well used) 315hp/400 ft-lbs truck?
I'd sell it and buy a Submarine
I'm kidding, but jokes aside -- the fuel issue is a non-issue for me in my use case, and the use case drives the decision on what truck to buy, and midsize trucks fit a use case that full size do not. I don't run larger tires - never needed more than 32s to get anywhere - and for towing, my current rig suits me just fine with it's 7k tow rating. While it certainly does drink the fuel when towing, I don't do that very often for it to matter enough for me to trade my truck for a different one. Two jerry cans -- which are cheap and easy -- puts me in the mid 20's you identified in terms of the amount of fuel I'm bringing along, and I rarely need it. When I do, I have it.
If I was towing a lot, I would get a different vehicle because my use case would be different. And I'd likely skip the half-ton class and go straight for the HD series. If I was wanting to mount super big tires, I wouldn't be going with an IFS rig because it's way easier to go with big tires on a solid axle.
I don't know why Ford spec's the cars the way they do - maybe it's to avoid cannibalizing from the F-150 (or with Chevy, the Silverado). Maybe it's CAFE standards. Maybe its' engineering/crash ratings -- there could be a lot of things driving the decision and it's likely a combination of things. But, the mid-size fits my use case better (as it does for a lot of folks), hence why they sell like hotcakes - it's partially empty hype, but it's also significantly that they do a particular set of jobs in a particular way that works better for a lot of folks.
My point is - my use case means for sure a compromise on some things, but those are very liveable and not deal breakers. The full size trucks represent a compromise too -- but that compromise is a lot more expensive and/or impossible to address, and to me that
would be a deal breaker. I cannot make a full size truck more narrow, which for example last weekend, would have meant skipping this part or destroying the environment on either side of the trail, which I am not willing to do.
