Frontal area and DRAG are the biggest factors unless you are pulling grade.

For example:
Empty 7x7x14 box trailer (1500ish #) pulls harder down the highway than my boat (3500ish #) with nearly the same frontal area but a lot more streamlined.
Or, a car trailer with 4000 # of lumber on it will tow better on the highway than that cube trailer.
Then there is the new box trailer that is not as boxy but bigger, tows better than the old smaller trailer.

Find the most aerodynmic trailer on the market. It makes a huge difference. The weight will hurt starting and stoping but once it is roling and you are on flat ground it is all wind drag.