To make it clearer, if the user wants to install oo-base at a later point with the currend Mdv model he would have to download 20MB if there has been no security updates since release, or 70MB if there has been a security update in the meantime.
FYI, there is currently a discussion on the LibO discussion list this very point. You may want to join in and have a say. There is talk of an incremental update or other methods.
They are on Gmane too! Marc