When Hibernate loads an object into a Session it creates a state snapshot of the current database state of the object, so that it can perform dirty checking against the snapshot.
As a read only object will never be modified, this snapshot is not needed and memory can be saved.
M. Lochau, S. Mennicke, H. Baller, и L. Ribbeck. Leveraging Applications of Formal Methods, Verification and Validation. Technologies for Mastering Change, том 8802 из Lecture Notes in Computer Science, Springer Berlin Heidelberg, (2014)