database is bigger after dump/restore - why? (60 GB to 109 GB)

Hi. Last week our 60 GB database (per psql \l+) was (re-)replicated to
the DR site using SlonyI, and arrived 109 GB in size which caused a
problem as it filled up the filesystem on the DR server - we expected the
DR database to be the same size. Mystery.

Now just past weekend we upgraded our production server by pg_dump
and pg_restore, and again the database is 109 GB in size!

Most of our data is in a single table, which on the old server is 50 GB in
size and on the new server is 100 GB in size.

Could you please help us understand why a COPY of the data into a new
database (whether DR or the new server) results in different disk usage?

Somebody mentioned on the Slony users list that there is a kind of padding
that goes in that actually helps performance.

Is there a way to track disk usage MINUS the padding?

Thanks,
Aleksey