Are you using Odoo V7 or V8? I'd assume that your Odoo is saving all its images in your postgreSQL database. Due to all the images being saved in your database your database size grows fast. For more information look at https://www.odoo.com/forum/help-1/question/where-are-document-attachments-stored-529 for example. You'll find out more about this here.

i apply this requete  for knowing what is the table responsable of this size:

SELECT table_catalog

pg_relation_size(table_schema || '.' || table_name) As Taille_donnees

pg_total_relation_size(table_schema || '.' || table_name) As Taille_totale

FROM information_schema.tables

ORDER BY Taille_totale DESC;