Sunbeam says:
but... I don't totally understand wanting the tow vehicle to be heavier than the load. I mean, in a way I do, because mass is good. But on the other hand something like a 1-ton dually diesel truck is a super capable towing machine, and I would think it would often be towing a load heavier than it is.
Yes, often folks tow heavier loads than the truck. I had my first experience with this when I worked in the High Sierra, driving a tractor (with three driving axles) towing an 80,000 lb D8 cat mostly off road, and up and down some very steep grades. Of course that load drove very slowly.
I have driven some boats which weighed more than the truck--the TomCat being one. All is well as long as you are pulling, level, and no serious cross winds. Once the load is pushing, or swaying, you do notice the difference.
All too often people who own trucks totally
ignore the manufacture's instructions, that over a certain load limit, that either a weight distribution hitch (preferably with anti sway bay), or a 5th wheel or similar hitch where the weight is over the rear truck axle is required over a certain weight!!
For example my Yukon has a sticker: weight carrying hitch up to 5,000 lbs, and tongue weight to 600 lbs. Up to 10,000 lbs, and 1000 lbs tongue weight requires a weight distribution hitch. I found that with my Ford Diesl Excursion (heavier than the Yukon), a weight distribution hitch improved the towing characteristics. Same with going to E/H brakes on the C Dory 22 trailer.
Although a lot depend on suspension, factory and after market anti sway/shocks, air bags, etc, the heavier the truck, the less sway, and "Tail wag the dog" there is. Just my observations from towing trailers for over 60 years.
We of the C Dory world often do overload our trailers, and the vehicles we are towing with. Not many accidents, probably because we are a group of fairly careful drivers. But I have seen some bad wrecks because of trailer instability.