As of September 2010, HP has updated HP 564 as well as several ink cartridge models (178, 364, and 862) to be a smaller size than their originals.
The updated black XL size, in particular, is noticeably smaller and does have a lower page yield but with a reduced price to make up for the change. It will still be installed in the same spot in your printer as the original size HP 564 ink cartridge.
Below are images showing how the old cartridges fit versus the new, respectively.
