question about time machine

ok so i have had time machine set up for a little over a month now, i dont normally keep it plugged in all the time since its syncing my mbp. randomly a couple days ago i noticed that the harddrive space on my computer is down by 13 gb and no one else uses this computer and i havent put anything on it that should take up that much that i can remember. so when i did a backup on my time machine today (its been about a week since my last one) it was even syncing an extra 13gb. i dont know if there is a way to compare two backups from time machine to see what the differences are or if there is anything else that i could try to do to figure out what is taking up this space. i ran into this problem once before with leopard except it was larger than that i could account for 20-30 gb of space like it was taken up by something

any help or suggestions are greatly appreciated