Difference in DB size with dump and pg_database_size

Hello,
any one can explain me why I have difference between db size when I
dump db, I see it's 5G and when I run SELECT
pg_size_pretty(pg_database_size('somedatabase')) As fulldbsize; on my DB
postgresql return: 10 GB

I run vacuum on db every night. Why is that huge difference in size ?

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Regards,
Condor