I found that after upgrading to Office 2013, that I had lost quite a bit of disk space, but did not know where. I used the TreeSize Free tool to find it.
Below you can see that the OneNote\14.0 folder is using over 9GB of disk space (a lot out of my 120GB SSD in my laptop). Given that I was now using OneNote 2013 and the older OneNote 2010 was removed by the upgrade, I figured I could delete the entire 14.0 folder.
Naturally I made sure I had full backup of my laptop before doing this, but after deleting the folder, I’ve seen no ill effects at all and was able to reclaim over 9GB of space.