You should get a message like:
File not found for 68f31baf-2e06-4871-9507-841838c995ad: LocalFileSystemStore:: File does not exist for key: 68f31baf-2e06-4871-9507-841838c995ad. Setting size to 0.
I had some problems with the file size populating job on 7.5 (or .4 maybe) that were related to events, but events should be disabled in 7.6.
I do wonder though if it is a rare case. We already created a ticket when we did a test migration of a client of ours. Locally the modeler would run out of heap space due to this size calculation event.
Regards,
Ronald