Got the measurements now (also added to OP):
Small PCB 7g
Bottom shell 20g
Top shell 55g
All together with screws 112g (my guess is there are rounding errors for the low weight parts, it's just a kitchen scale)
I don't see any real potential for weight loss here, I mean you can butcher the mouse of side buttons and wheel to go down to 100g, but you'd have a two button mouse. The top shell seems really heavy in comparison, although it doesn't...