If there are tables in the tablespace, then the tablespace is not empty even though the tables are. If you need your tablespaces to be smaller, you need to make your tables smaller. The 'deallocate unused' might help but I suspect it will not get you as much as you want.

You could drop all of your empty tables, coalesce the tablespaces, and then build your empty tables again except with a small initial extent (and normal sized next extent). Then you should be able to resize your datafiles so they are not quite so big. When the time comes that you need the space then you can make your datafiles big again.

-John