Schema size [PG TOAST] - PostgreSQL 9.2
The function pg_total_relation_size() returns


  Total disk space used by the specified table, including all indexes and TOAST data


Given the disproportionate size between the tables and total relation size (100 to 1), it is probably mainly due to TOAST data. TOAST data is already compressed (LZ), so it will be unlikely to be able to compress it any further.

If you have a lot of duplication, or want to store blobs outside the database, you could resort to this: Storing blobs outside the database.