pg_dump file size

I'm a newb  running Postgres 8.3 on a SuSE 7.2 system. I have a database whose size reports as 644 MB using

select pg_size_pretty(pg_database_size('adempiere'));

When using the following command to back up the database,

su postgres -c "pg_dump adempiere | split -b 3000m - /tmp/adempiere_`date +%F`.dmp"
 
the result is a 49.5 MB file. Curious as to how the backup file can be less than 1/10th the size of the database?
 
Thanks,
 
-Mike