If you install the free "Microsoft Search Server 2010 Express" using the free "SQL Server 2012 Express" as the back-end database, MSSSE 2010 still thinks that the database size limit is 4 GB even though it really is 10 GB with this release of SQL Server Express.
The trick is to add an undocumented registry key on the MSSSE server and set it to 10 GB:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\14.0\Search\Global\MaxSearchDBSizeMB of type REG_DWORD and set the value to 10240 (decimal).
MSSSE will now be aware that the database limit is 10 GB.
Thanks to Trevor Seward (MCC) of the TechNet forums for pointing me in the right direction !
Link to the forum thread where you can see the offending SharePoint code: http://social.technet.microsoft.com/Forums/en/sharepoint2010setup/thread/870dde9a-ea60-4133-82fd-f6d25385f035