We have set up a second Exchange server within our site and moved 170 mailboxes over to it. However, it does not appear that the mailbox data has transferred. The Private Information Store is only 15MB in size. Did we do something wrong?
Without knowing what technique you used to move the mailboxes, or how big the mailboxes were before you moved them, it's hard to say.
It should be easy to tell if the mailboxes are empty or not. Just find that server in your Exchange Administrator and go to the Private Information Store > Mailbox Resources object. You should see a list of the mailboxes. Among the columns displayed (you may have to scroll right) are Total K, indicating the size of the mailbox; and Total Items, indicating how many items are in it.