Q: Time Capsule won't backup 2nd computer.

I have a Time Capsule 802.11n (3rd Gen). I have been backing up my Macbook Pro for over a year.  Recently when I tried to backup my wife's desktop Mac, Time Machine gave me this message: "This backup is too large for the backup disk".  Time Capsule contines to accept new backups from my laptop by deleting earlier data. Any suggestions how to handle this?