Anyone who’s been using Time Machine for years knows that your Time Machine drive eventually fills up. Time Machine handles this situation by automatically deleting your oldest backups. If you don’t want this to happen, the best solution is to buy an additional hard drive. Storage is extremely cheap now, with 4 TB external drives selling for as low as $130. In a pinch, there are some things you can do to reduce the size of your backups.
Folders or individual files can be excluded from your backup by clicking on the Options button in the Time Machine preferences panel. Keep in mind that items added to this list will be excluded from future backups. Your old backups will remain on the backup disk, at least until they are purged during Time Machine’s thinning operations.
To forcibly delete folders or files on your backup disk, enter Time Machine and right-click on the item you’d like to delete. You’ll see an option to delete all backups of that item. If you go to a specific date in the past, there will also be an option to delete just that snapshot’s backup.
GrandPerspective is a useful tool to see which files are taking up the most space on your drive. You might want to run this on your primary drive, as a full scan of your backup drive will take a long time due to the large number of files.