diskspace usage recovered on db rebuild

Greetings worthymen.

I have a question regarding filesystem disk space usage.

We have a production database containing 5 years of sales data.
Linux 2.6.5; Postgresq 7.4.7.  VACUUM ANALZYE the entire database
everynight (about 40min).
It's size, @SUM(pg_class.relpages) * 8192K, is ...

  About 66 Gigabytes on disk.

When I rebuild the database (using pg_dump and pgsql ), the new
resultant database is ..

  About 48 Gigabytes on disk.

A 27% space savings.

Can someone tell me why that is?

Thank you,
David Link
White Plains, NY
Long live Postgres.