1991 Ls400 Rear Suspensions Help Please?

[Salacak]

If I put 100 pound in my back seat or in to trunk this car goes down a lot? Why is that (I don't think I have a air suspensions), I vent to mechanic and got advised that springs are too weak and has to be replaced!!! Didn't make sense to me but hey we listen to these idiots all the time.Changed the springs plus the new struts same garbage again, little weight and car goes down.

What am I missing here.

Please help.

Thanks