Unable to determine what has a particular OID

Hi,

We currently have a DB (8.3.7) in production that seem to be taking up more
space on the HDD than was anticipated.

After looking into the the data directory a particular file/oid is around 21
GB (the oid has 21 files), this OID can then be found in the pg_class table,
the explanation for the size being that the table has around 12.5 Million
rows. During the inspection of the data directory I observed another OID
that is around 8.5GB, but I cannot locate the corresponding entry in the
pg_class table (even after dumping the table into a file and grep-ing for
the OID in question).

Any other ideas for finding what the mystery OID is ?

Thanks in advance,

Andy