Most 7k trailer have between a 4,800-5,500K useful load (7k being the max GVWR - trailer + load). The 14K dump trailers depending on bumper or GN style, only have about a 8-11K useful load, depending on the model. Once you figure in the weight of the double frame, the hydraulic equipment, cylinder, etc, that all adds up quickly.
Farm usage is one thing and there are things you can generally get away with on county roads, that will NOT fly on many public highways and interstates. The County and State guys are trying to make people safe, but it can be a PITA for Joe Farmer or Sawyer just trying to get crap done.