Inside the InstallESD.dmg was a BaseSystem.dmg — a minimal bootable recovery system. Mavericks used a (not yet lzfse named, but an early variant) for some of the system files inside the DMG, making the image smaller than Mountain Lion’s while maintaining boot speed.
If you downloaded Mavericks ever when it was current, it is tied to your Apple ID. mavericks os x 10.9.x dmg