Why are .sqlite files so much larger in Firefox 4 ?

[Bluefang]

Actually, shrinking and growing the file size can cause both file fragmentation and internal database fragmentation. If there is always available space in the file, it will reduce the potential table fragmentation when adding new records.

This is also why constantly VACUUMing the database is actually not a good thing. Having empty space in the file, especially when it's spread amongst the different data structures, allows those data structures to be updated and grow in a more contiguous manor. An occasional VACUUM is a good thing, and Firefox should already be doing this automatically. But doing it too often gives you diminishing returns and can be unhealthy for the database.