Picasa's "overhead" for its database tends to be somewhere around 1.5 percent of the size of the photos being worked with. As an example, if you have around 95 GB of photos, your Picasa Database (the "overhead") would be about 1.8 GB.
If the Picasa database gets too large to keep it at the standard location, you can move it using "Tools"/"Experimental"/"Choose database location..."