Roy's chair is fairly heavy, 370g. Way heavier than I expected it to be, and I felt the weight of it even in the box. That's why I was surprised it wasn't magnetised. I figured with that heft, there had to be something in it other than PVC.
The other two chairs are much lighter. Ozy's is just under 270g, and made from hollow PVC. It's textured to look and feel like something other than smooth plastic, but it's not anything genuinely stupid like polystone.
Sanji's is the lightest by far, 100g.