Some bridges are more spaced from bridge pickup than others, the scale length determines this? Thanks!

Mostly the scale length.  With a custom made bass, you have some flexibility

If you are asking about Bridge position itself, it is its distance from Nut that determines Scale Lenght (and fret positions by consequence), since this is related to how much string is free to vibrate between Nut and Saddles.
 
(Message edited by mario farufyno on April 24, 2012)

But you have to be aware that Saddles hardly lay exactly at scale lenght end (lets say 34? from nut) because differences between string gauges prevent them to intonate correctly until we move it slightly sideway. So you have to place the Bridge in a position that gave some space to move saddles back or forward when needed.