Agree with all the above. The RV marketing people bandy around terms like "ultra light" without any real meaning. However, any trailer that weighs less than another of the same size obviously has had some weight trimmed off it somewhere. Sometimes that can be done by substituting lighter but still strong materials, e.g. aluminum instead of wood or steel, but I would not assume that to be the case without investigating. It is also possible (likely even) that some things were simply made lighter without any real regard for long term reliability.
Obviously no trailer with a gross (not empty or dry) weight of 4800 lbs is not going to be large. Whether it is rugged or not depends on whether the weight was employed for sturdy chassis and structure, or for extra length and fancier furnishings. You can only get so much in 4800 lbs of GVWR, and at least some of that has to be reserved to carry your personal gear.
If you look at two trailers, each with a GVWR of 4800 lbs and an unladen weight of 3800 lbs, and one is 20 ft long while the other is 17 ft, you can bet the longer one was skimped on the trailer frame, wall and floor construction to keep the weight down.
A word of warning: ignore any sales brochure, website or sale person who is pushing an RV with a dry weight at or near your 4800 lb limit. You need a trailer that weights less than 4800 lb when fully loaded. I'd suggest a trailer GVWR limit of 4400 or less, to make sure your loaded weight is within the tow vehicle capacity.