All cPanel WHM accounts larger than 1.7gb create a backup that is always 1.7gb - even if account is 3 times that size

I've noticed that when I create a backup for any account in my whm (or in cPanel user account) it will always be a tar.gz of only 1.7gb if the unzipped account is very large. For example I have an account that is 1.785 gb in unzipped size and the backup is 1.7gb. BUT... I also have an account that is 4.197 gb in unzipped size and that backup is also 1.7gb. So is cPanel backup failing to create a full good backup for all large accounts? How can all the files for a 5gb account fit it a 1.7 gb tar.gz zipped file