There are a number of threads on RAS concerning vehicle size vs sway potential. The gist seems to focus on mass and wheelbase of the tow vehicle (more mass, longer wheelbase preferred), fore/aft loading of the trailer (10% of trailer weight on tongue), tire size/construction (radial vs. bias ply) and tire pressure for both the trailer and vehicle (higher pressure = stiffer sidewalls).

Of course, since this is RAS, assorted self proclaimed "engineers" will come out of the woodwork arguing the exact opposite of any of these principles.. Using their logic, the "ideal" tow vehicle is a motorized skateboard with silly-putty tires and the ass end of the trailer loaded down. But with vortex generators. Please ignore these people.