The obvious advantage with the slide in film holders is that it's more convenient than switching the film back and GG back.
As to size advantage, I think it's significant. I have a 2x3 Century, a extra lens, a film back, a few sheet film holders, and a meter all in a bag you'd expect to use for a 35mm system. You couldn't do that with a 4x5 unless using a much larger bag. I don't have anything here to measure, but I would guess that a 2x3 press camera is about 1/3 the volume of a 4x5 press camera.
Also, with respect to Graflex type backs (which were made in 6x6, 6x7, and 6x9), there is an outer shell and an insert. I've seen them mismatched where the shell doesn't match the insert so, if you buy one, make sure they match. There is also a whole discussion around film flatness with the Graflex backs with some models supposedly better than others. You can Google and learn about this if you go this route.