I originally went to six string because a big hero of mine at the time did. I worked at a store at the time and got to go to NAMM, and also talked with big builders of the time. And read all my heroes' words that I could find. Almost all agreed, told me, etc, that mass equaled tone, sustain, and affected other things as well. I grew up believing it, believing I experienced it, and still believe that way. I went after basses with mass, and Six String basses had more. I also preferred wide spacing at the bridge. And bigger bodies, necks and headstocks.
Not all believe as I do. And today, with the improvement of electronics and construction techniques, maybe mass isn't as big of a deal as it seemed to be back when. But I still prefer my basses have a little weight to them. I've gone back to 5s, now. But I still play Big 5s. My main is a Jazz 5 that weighs a little over 11lbs.