Yes and no.

MIrroring doesn't copy the trash folder (by default), and it doesn't copy some log files too. And some indexes may only exist where they have been used....

However, generally a discrepancy  that large would make me suspect something bad.

The first step is

    tellmail resync

after that's completed see what it looks like, if it still doesn't match use du to find the folders that don't match in size and see what's different.

    ChrisP.

Should I expect /var/surgemail to be the same size on server and mirror? I see a huge difference with the “du” command.

server:  11414664K	/var/surgemail
mirror:   5328388K	/var/surgemail