The website reports that sometime in February, Sony will release a system update for the PS3 that will increase the hard drive space necessary to run the PlayStation 2 version of FFXI. PS3 models that feature 60GB and 20GB drives are the only units being affected, but it's worth noting because the FFXI client will require an additional 9GB of space after the patch (21GB total).
The PS2 version of FFXI isn't changing, nor is the Windows version, so don't sweat it if you're not playing the title on your PS3. If you are, though, the source article is worth a read. As of press time, Square-Enix hasn't divulged a reason for the space increase.
[Thanks to Andrew for the tip!]