PostgreSQL relation size does not sum up

I have a very large table, with blob fields, called data. I've tried to figure out why it isn't cached well and repeated SELECTs are quite slow:

=> SELECT pg_size_pretty(pg_total_relation_size('data'));
157 GB

This seemed a bit large, so I tried to sum up the data:

=> SELECT pg_size_pretty(pg_relation_size('data'));
19 GB

With the indices:

SELECT pg_size_pretty(pg_relation_size('data_pkey'));
757 MB   
SELECT pg_size_pretty(pg_relation_size('data_file_end_date_idx'));
766 MB   
SELECT pg_size_pretty(pg_relation_size('data_file_end_date_idx'));
766 MB
SELECT pg_size_pretty(pg_relation_size('data_merged_idx'));
854 MB
SELECT pg_size_pretty(pg_relation_size('data_owner_idx'));
794 MB
SELECT pg_size_pretty(pg_relation_size('data_session_format_idx'));
779 MB

The summation of the data and indices size is around 26 GB, but the total relation size is near 160 GB. The table was restored from dump just a while ago, and no writes were made since.

• What's the explanation for this difference?
• Any way to reduce wasted disk space? Will clustering help?