Time Machine is not a 'straight backup' or mirror of your hard drive -- it is a rolling backup. Every hour / day / week it snapshots your drive and records the changes. So if you download (hypothetically) 10 gigs of porn on Friday night while Time Machine is running, it will back up that data even if you delete the movies on Saturday. It will keep Friday's snapshot of your evening... so your Time Machine volume will grow much more quickly than your hard drive since it records changes and not just mirrors.
Eventually TM will run out of space and start by deleting the oldest snapshots. If you have data (like your porn directory) that shouldn't be covered in the rolling backup because it changes too much and you don't care if it is snapshotted, you can exclude it from Time Machine.