Set up differences/jazz and classical players

[Greg Clinkingbeard]

When I bought my Upton, I told them I wanted it set up for jazz play with the strings close to the fingerboard. I wasn't experienced' enough to give any more information regarding string spacing or radius. The bass arrived almost exactly the way I expected although I might prefer less scoop in the fb and I did cut the nut slots a bit deeper. My teacher commented on the nice setup. Since his main bass was set up by Bob, I take it as a compliment. Aside from the sheer size of his bass, it's got an incredible setup and tone.

Question, Why do basses vary so much regarding tension? Is it the tailpiece , the thickness of the top? What is it?