Time Machine - Something is making it back up too much

Q - " I have Norton 360 on my Macbook "

The Computer issue in your Question above is Directly related to the below reply and advise given IMHO.

A - There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

The Only thing this Antivirus software is protecting is the Bank Account of the Developers and for zero return to the User aside from the problems this software creates.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and comprising the Operating System

It may require a Reinstallation of the OS in-order to replace any modified, altered, removed or corrupted elements of the OS this software has inflicted on this computer 

Any of the below should be removed as per Developers Instructions 

This will included Norton Antivirus 

There has been enough advise / suggestions offered for the User to make an informed and educated choice what the next course of action is required.

LVN46 wrote:

I use TM with an external USB drive that I plug in every night before going to bed. The initial backup is small and quick. But when I wake up in the morning TM is working on a new backup and is only about 30% finished and seems to be backing up about 4GB of data. What could be causing this? I use Blackblaze but it runs all the time and if that was the problem the initial backup wouldn't be normal. I have Norton 360 on my Macbook but, again, it runs all the time and doesn't affect the initial backup. Any ideas? Thanks.

I would uninstall /disable Norton and compare your results.

Uninstall all third party apps that are Cleaners/Optimizers/Anti-Virus/VPN

all known to cause issues on the macOS

Third party AntiVirus is not recommended— it typically does nothing but add issues to the macOS and competes directly with Apples own built in security:

macOS - Security - Apple macOS - Security - Apple

Apple Platform Security - Apple Apple Platform Security - Apple Support

more—https://manuals.info.apple.com/MANUALS/1000/MA1902/en_US/apple-platform-security-guide.pdf

Effective defenses against malware and other threats… - Apple Community

Effective defenses against malware and ot… - Apple Community