@
gingerbreadman - try this, open the book folder (press 'O'), drag and drop the pdf onto the book details panel - size should update to size of the now largest format.
The size comes from what's recorded in the database rather than dynamically interrogating the file system, to do the latter would have a deleterious impact on calibre performance - particularly on thumb drives, a NAS, or Internet file storage service like dropbox or onedrive.
I don't know of any way to automate - maybe a "Recalculate size" option could be added to Bulk metadata edit or a format agnostic optional plugin.
Have a look in the
Job Spy plugin, its the sort of thing DaltonST may have included, or something he'd be willing to consider.
BR