I think there is a algorithm for calculating SPDS disk space. I would say that SPDS tables require a bit more, but generally not more than 10%.
If you chose to have BACKUP=YES an additional 18 bytes will be added to each record, and if you have a lot of tables with a short record length, that would probably build your tables more than 10%.
But this has never been an issue in the projects I've been participated.
/Linus
Data never sleeps