Correct me if I'm wrong, but I thought the whole point of the treemap display was that the rectangles representing the folders were proportionally sized, so the more disk space a folder users, the larger it would be in the treemap.
However, I'm seeing results like this when trying to work out where all my space has gone on my games drive:

Notice that the top-left item, Roberts Space Industries, is 66.8 GB, yet its rectangle is about the same size as the 2.2GB Silent Storm below it (which uses 1/30th of the disk space), and not much bigger than the 414MB Bomberman to its right, despite being 150 times bigger in terms of disk space.
Have I completely misunderstood the point of the treemap? In which case, what is it?
Or is this a bug?