Comment 4 for bug 1240373

Recent change to make size immutable (except for administrators) is the reason.

commit 9ae9c09e03d68338ac7ec342982489db3f5f54ab
Author: Stuart McLaren <email address hidden>
Date: Wed Aug 7 06:22:12 2013

    Make size/checksum immutable for active images

    Once an image becomes 'active' it should only be possible for an
    administrator (and not a regular user) to change the size and checksum
    values.

    Addresses bug 1209148.

    Change-Id: Ic65afbca4a4161dfa77ce0a296a1bd72d96902ca