tables for IDOC's

Some time ago I created a job which deletes all idocs older than 1 year.

I am asked now to check what is the benefit in term of space(tables, database..)

I wander in which tables idocs are stored(if tables are different for different idocs could you let me know tables for most commonly used)

. However I am afraid (having oracle database) I can not see the benefits as segments/database just grows regardless of empty space.