Broadly speaking, fifth wheels on average do tend to be bigger and heavier than travel trailers. There are exceptions, of course. I guess if you consider bigger and heavier as more "up" then speaking of moving up to a fifth wheel from a travel trailer makes some sense. Similarly, with that criteria, one would also generally be moving up from a class C motorhome to a class A motorhome.
It's certainly not true that larger is always more satisfactory. That depends on one's particular needs, wants, camping destinations, and so forth.