Time machine does more than just backup your data. It doesn't just snapshot the current state of your data, but it's historical changes. So you have upgraded to a new version of OS X - Yosemite. There will be new extra files but it's not just say 10Gb worth of extra files, it might be 35Gb of changed files, and 5Gb of new files, resulting in a 40Gb change for Time Machine.
This way, when you restore, it's not just a case of being able to restore to the current most recent backup on your TM, but being able to go back in time to any point that it still covers and restore to that point, which may be as far back as prior to OS upgrades etc.
I hope this helps.