Personally I've never found a bass too heavy but then I've had a 60+lb toolbag hanging off my left shoulder for years.
I do have trouble with Sciatica, interestingly it only flares up when I've been standing or walking for long periods of time carrying NO weight on my shoulder.
So in my case the back muscles need to be working,I've shoulder straps fitted to the toolbelt too so the weight's not hanging round my waist.
I'm not suggesting running around with a cement bag to tone up but atrophy is common with unused muscles and might explain the need for lighter and lighter basses
with some folk.