more sustain and volume higher up on the fretboard?

[tranceFusion]

Why would a bass have significantly more volume and sustain the further up the fretboard (closer to the bridge) that it is played. I mean, I definitely hear differences in the overtones the shorter the string becomes, but on this bass it is just ridiculous.

Maybe the strings? It is strung up with D'Adarrio Chromes.

[GeoffByrne]

High action? Holding down the strings pushes them closer to the pickup? High action also means cleaner & better sustain while held down?

[iamthebassplayr]

I would guess that, like Geoff said, pushing the strings down is causing them to come much closer to the pickups. That would cause an increase in volume and a more audible sustain.