Where can I find the files using up my external hard drive?

I have a 1TB iMac and have two external hard drives attached to keep the computer free of hard drive use for two reasons: Disk 1-is a 250GB hard drive I have dedicated to perform Time Machine backups of my home folder. Disk 2-is a smaller 120GB hard drive I keep my compete iTunes library of music in case the computer has a malfunction, so all the work I've done putting the library together will be secure. Here's the thing...the iMac's hard drive is hardly used, just as I want containing about 150GB total. Meanwhile, the Time Machine disk is nearly full with 225GB used, and just 25GB remainibg. The hard drive with the iTunes files looks pretty good too using about 70GB of it's 120GB. I believe Time Machine is in no way, backups the iTunes library just the iMac.

How can I figure out where the additional space is getting used up on that disk? I do have a couple of extra logins I use for alternate use such as a different iPhoto library I want kept seperate from mine, but I did not think the backup was saving those files as they are not even under the Time Machine setup.

Can anyone advise why the backup is larger than the use of the computer? I knew when I dedicated a smaller hard drive to back up a larger one that it was possible the backup drive would get too full, but I can figure out what's happening. I've erased the Time Machine disk and allowed it do a completlely new backup and again, it's always sigificantly larger.

Thanks!