The BTree Page Distribution Dialog gives a visualization of the space used by just the two BTrees (the NBT and BBT pages) of a PST file. It does not include space used by blocks.
The upper left pixel represents byte zero in the file; the lower right pixel is the EOF byte. Each pixel represents a variable number of bytes, depending on the file size (as indicated by the title bar). If a pixel has a NBT page mapped to it, it is colored
blue. If a BBT page is mapped there, it is colored red. If both are there, it is colored black.