PGDATA directory 50% smaller then size give by select pg_database_size()

[Finely]

What are the high level reasons for this assuming
we just have one database call it "test", in addition to the postgres one.

If I reload "test" from a fresh pg_dump will the
sizes be closer. Is this purely due table bloat?

What exactly is included in pg_database_siz e?

I could not find any documentation on discrepancies
between the actual size of the data directory and
that reported by postgres.

Thanks in advance.