Make sure your external drives (including any removables that may be mounted at the time, like camera cards, USB keys or iPods) are listed in the exclusions in the Time Machine preference pane.
Also remember Time Machine is making and keeping MULTIPLE backups, not just one copy of your drive like Super Duper or Carbon Copy Cloner. Time Machine is NOT designed to clone your drive but to enable you to go back to any of MANY backups it has automatically created. Therefore it is always going to need more capacity than your drive.
I read a while back that a guideline is 2:1, meaning your Time Machine drive should be at least twice the size of the drive you're backing up.