If something is twice the size, you could say it's 200% (2x) the size, or 100% (1x) larger.
So conversely, if something is half the size you could say it's 50% (0.5) the size or 50% (0.5x) smaller.
100% (1x) smaller would be 0, i.e. you've taken away 100% of what you had.
1.8x smaller would be therefore be an imaginary -0.8x the size of the original.