The size of the tablespace doesn't change when I import a smaller dump or deleting old data
Tablespace size is not changed by creating, dropping or importing tables (if we forget autoextend option). If your tablespace has no data after droping tables, it is easy to resize datafiles or recreate tablespace. If not empty tablespace, after tables drop, you can try to resize datafiles ALTER DATABASE DATAFILE 'file1' RESIZE 20000m.  Note that resizing may not always be done if your tablespace still contain some tables/indexes. In this case you will need to do some other actions before resizing... (alter table move or drop+import or...)