You can test it by downloading a large video or something then deleting it, then emptying the trash. That moves the file to /.MobileBackups.
Then run this command in Terminal to see this size of the folder.
Code:
sudo du -hs /.MobileBackups
Then do a TM backup and check again and you should see the size is unchanged. As long as there is plenty of drive space, it just keeps on saving things to that folder.
https://support.apple.com/en-us/HT204015
More info on how it works at this link.