Size of an "empty" PRIMARY filegroup
you can use shrinkdatabase command to reclaim the freespace,
But be aware ! shrinking in many cases causes fragmentation , you can rebuild  or reogranize all indexes depend on fragmentation percentage. 

DBCC SHRINKDATABASE(N'YourDatabaseName' ) or 
DBCC SHRINKFILE(N'YourDatabaseName', Size )


After that, you should change your default filegroup from primary filegroup to another one.