I think the height of the towed vehicle on the trailer is much more of a factor than a bit of weight. Now in my case, comparing a tandem car hauler (2100 lb) to a single axle utility trailer (900 lb), both carrying a 2600 lb samurai, the weight does make a difference. But in most people's cases, with larger rigs on the trailer, the weight of the trailer will not make as much of a difference.
Now to aero, I can tow my Tbird (3900 lb) on my car hauler (2100 lb) and a couple of mpg's more than towing my Samurai (2600 lb). Towing 1300 lbs more, that is no higher than the roof of my truck, is much easier on the truck, than the lighter load that the hood is level with my roof of the tow vehicle. Stop and go, the weight makes more of a difference, but at highway speeds, the aero is the larger factor.