Try hooking a large trailer on the back of a small Toyota pickup. Load it to where each axle is not over loaded. This means you will be within the GVWR.
The problem is the truck is not heavy enough to handle the trailer/load weight. It will start fish tailing and you will loose control.
GCWR is often over looked in most inspections. In fact a lot of manufactures do not even give this rating
But go over board and you are asking for trouble and not only from the DOT officer.