Engine size is not the answer. The issue is the trucks frame and drive train (aka drive axles, differentials, suspensions etc)
Tundra is nothing but a beefier Tacoma with a V8 car engine. You can not compare a 3500 Dodge/Ford/Chevy Truck with an real 1 Ton Diff. in the rear(the size of a small engine) with that puny thing the almighty Tundra offers (that's a 4Runner rear axle).
Again call me when Toyota makes a truck that can move at 18,000 LBS GVW and tow in excess of 15,000 LBS at the same time.
That would be found in Toyota's heavy truck division. Hino
I do believe that there will be standard and heavy duty offerings. Obviously you can't tow 15000 lbs with a Ford F-150 or Chevy 1500. You would need model that was built for extreme application.