As an example, a non-Unicode SAP table (DBTABLOG) in an Oracle 11gR2 database (character set ALE32UTF16) without any Oracle compression, was ~80GB in total size (sum of table segments, excluding indexes).
Once this was exported to disk using R3load export, in preparation for a conversion to Unicode, it occupied ~70GB in the data files (DBTABLOG.00*).
Once this was re-imported into a new Oracle 11gR2 database with character set UTF8 (again, no compression) it occupied ~90GB (sum of segments, excluding indexes).
You must remember that this table is specific in it’s usage. It doesn’t have any rows deleted from it, it’s append only, so it should grow in a nice uniform manner and not be fragmented. There may be other tables where you could save space.
If you notice that your R3load export files are significantly different in size compared to the Oracle size, then you could have some serious fragmentation inside your Oracle database.