An example of the problem with compress=y

If you have a table with 200 2Mb extents but it only contains 10Mb of data when you export the table with compress=y Oracle to take a sum of the total extents and use that for the intial value.

Hence when the object is recreated with the import it will be 400Mb in size to store 10Mb of data. If however, you use compress=N the object will be recreated with 5 extents occuppting just 10Mb.

Regards