I think that's what has put me on the lookout for a lighter bass. I thought chambering might be the thing. Warmoth advertises that it's chambered bodies reduce weight by 25%... but that's 25% of the *body* weight, not the assembled bass weight. At best that brings most chambered basses down into "swamp ash" territory...a reduction of about a pound overall. 
By contrast my ES body basses (one from Pit Bull and one from Fret Wire) are MUCH lighter. The assembled basses are around 3.4 kg. My swamp-ash precision weighs in at around 3.9 kg. A Gibson Midtown bass (closest commercial bass to a PB 
ESB-4) weighs in at around the same as my swamp-ash precision--about a pound less than the PB equivalent. The primary difference? The PB and FW bodies have a paulownia center-block, whereas the Gibson has a maple center-block.
I built the 2nd ES to get a neck and weight that I liked, and it worked pretty well. 
I am now trying to get I neck and weight I like with the contoured comfort and simplicity of a P-bass. I think this will work, but I may have to accept the dents.