URD file size increases by double the size of imported mbox

[PureMoxie]

I just imported a PocoMail mailbox using the mbox import. The mailbox had a few messages, but one had attachments and was about 15MB in size (I keep attachments encoded within the messages so they are completely stored within UR). After import, my URD file had grown by about 30MB. A compact didn't affect the size much. The message itself is not visibly duplicated within UR.

Could this be a bug?