Does it has a way to compact the database size?

Dear All,

I use PostgreSQL 8.0.3 for store my data started on Sep'06. Today, I
use database_size('name') function for checking my database size and
found that it's about 1209715345.

I think that something maybe wrong on my database because I backup my
database everyday and the backup size is about 10 MB. So I restore
databse from my backup file then use database_size('name') function
again and found that database size is about 56642193.

I can said both 1209715345 and 56642193 are coming from the same
database but I wonder that why it quiet difference on size.

Any Idea?
And How can I control or compact my database size for make it smallest
as possibled?

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