database size much bigger than tablespaces on filesystem

I have a Postgresql 8.3 instance with tablespaces totalling on about 74G. This is fine.

But if I ask Postgresql how big my database is, I get a (unexpected) large answer: 595 GB.

This seems very strange. Disk I/O tests on the system are in the 'normal' range, but queries are slower than they used to be.

Is this corruption of the database? Or are there ways to 'fix' this oddity?

Btw:

I check the tablespace size by the os:

/usr/local/pgsql/data/tblspaces/du -c -h
74G total

All tablespaces are there. I check this by the \db command in pgsql

I check te database size like this:

select pg_size_pretty(pg_database_size('database'))
595 GB