The number of image versions in your projects will affect the size of a vault. The more versions you create per master, the smaller a vault will be in comparison to your library. Database files that deal with versions' thumbnails and previews are in the Aperture library but are not included in a vault. Restoring from a vault rebuilds these files dynamically. If your workflow involves creating a large number of versions, then your vault will be smaller in comparison to your library.
Plug-ins are not stored in the library but are included a vault backup. If you have a large number of plug-ins, your vault size will grow relative to the library.