As a kid I was fascinated with lead. We had a lead brick in our garage. I used to pick the lead up and be amazed at how much heavier it was then it looked.
These days I notice the same thing with clutter. When I'm working with a client and we fill up a trash bag, I'll lift the bag and be fascinated that it's heavier than it should be. I often have the client pick up the bag and notice the extra density. It boggles their mind.
There seems to be an additional emotional weight to clutter. All the experiences and expectations we have with and about our stuff adds weight. Our attention is powerful. Physics shows that our attention changes things.
I think that's one of the reasons why people feel so much lighter after letting go of clutter. That weight had been oppressive on their life and now it's gone.
Try it yourself and see if you can notice the difference.