I never quite understand this:

The previous DBA created DBs with export/import. However, when he did export from DB#1, he specified compress=y and import it into DB#2. Thus in the subsequent exports from DB#2, they all have big initial extents. I notice even if I export only schema from DB#2 with compress=n and import into other DBs, the result is still big tablespace, just like the source DB DB#2 (even though there is no data in it).

How do people actually reoganize the tablespace and shrink down the size if the tablespace doesn't really have much data in it?