Time Machine excludes a lot of files and directories you don’t need to be backed up by default. There is a file that contains all those locations:
Individual programs can also mark particular files to not be backed up. Typically, this includes caches and other temporary files. You can find a list of these exempt files by running the following command in the Terminal:
sudo mdfind "com_apple_backup_excludeItem = 'com.apple.backupd'"
However, you always have things that change frequently on your drive that you don’t need to backup. Here is my list of exclusions.