It's unique because in any other context, defining an element's size in terms of its size would be circular. But the transform property works differently: the rendered size is independent from the element's "real" DOM size. This can be seen in the fact that transform doesn't change the flow of the document at all; there's an invisible placeholder element that's still taking up the same space, and only its visual representation is changed. So the latter can be derived from the former without a circular effect. This is also the reason browsers can offload the transform property to the GPU.

Something I love to do with this is create "squishy" buttons that transform:scale(0.9) when under the :active pseudo-class (with a short transition set up for smooth animation). It's extra satisfying on touchscreens.