GoogeBrowserSyc save every session in a folder and that folder can be infinitely large!
My last cache was 120MB
Here you can see this Folder and this files can be delete.
Vista:
FLOCK
"%userprofile%\appdata\Roaming\Flock\Browser\Profiles\++flock userprofile++\browserstate-logs"
FIREFOX
"%usrprofile%\appdata\Roaming\Mozilla\Firefox\Profiles\++firefox userprofile++\browserstate-logs"
At Windows XP you have so search this Folder.