sometimes it may be necessary to reindex the HD on the mac so that it properly calculates what is taking up space on the drive. the following article tells how to do this:
http://support.apple.com/kb/HT2409
after this is completed go back into the storage information and see if the numbers have changed any.
also to check the size of the mail folder, open a finder window, then on the main menu bar (at the top) click on the word go and hold down the option key on the keyboard, (this makes the word "library" show up in the drop down) select library
now in the finder window it should show you all the files within your user library, find the mail folder and control click on it to get a drop down menu, from the drop down choose "get info" in the top right corner of the small window that opens it will show you the size of this folder,
if it is extremely large open the mail folder by doubleclicking on it and then look for something inside that says recovered messages and delete that.
regarding the ios backups: while you are in the user library, find the application support folder and inside of that you will see a mobile sync folder and within that are the backups from the ios devices. get info on this the same as the mail folder and see how large it is and decide how to handle accordingly.
let me know if you need any further help as i'm happy to assist!
