At first, I assumed that some form of corruption had happened. On closer inspection, however, I found the true culprit:
Nothing had happened to the iPhoto library or database. The increase in disk space usage was due to my finally getting an iPod that could handle pictures. ;-)
du -h | grep G
6.0G ./iPod Photo Cache
But - there's a lesson here that all software QE engineers have learned. When you think you've found a new bug, you have to examine the software under test for any changes, AND you also have to examine the environment in which the software runs for changes.
I like to look at software testing as an exercise in algebra. Your task is to take a very complex equation and find its answer. In that equation, you have some variables to resolve. The trick some times is that the variables may not be so obvious at first! Even if they are 6GB in size...