Frequent full backups by Time Machine on MacBook Pro M3

My MacBook Pro M3 has a 1TB HD and it only has 237 GB used.

My Time Machine HD (Western Digital) is 6TB.

It looks to me that every TM backup, backs up my entire system disk and the Time Machine HD starts erasing backups that were only done a few weeks ago to make space.

I recently upgraded from Sonoma to Tahoe & had high hopes that this was going to fix the problem but it has not.  I would have liked to have done a clean install of Tahoe but here in Australia, the full installer has yet to be added to the App Store, so I had to settle for an upgrade.  I am still hoping that a fresh install might fix the problem.

In the meantime does anyone have any suggestions?

A little more information….

My Time Machine 6TB HD is only a few months old, having replaced what I mistook as a faulty slightly smaller HD.  It is formatted APFS.

I only use my Mac for personal use, so not much changes on it from day to day, other than email, whatever gets cached when browsing the internet etc.  My data files are stored in iCloud Drive, but as I only have one computer I should imagine few files would be getting synched often.

I have run First Aid on both my system disk & the TM disk & they all come up error free.

I do not run any 3rd party apps like anti-virus, cleaners, VPN or the like.  I just have a very simple, vanilla out of the box set up.

Not sure what to do other than dump Time Machine which I have used for many years and love.  Currently I also use SuperDuper to do a clone of my HD monthly, and I have multiples saved, so not really worried about my data.  I just want Time Machine to work as it used to.

[Re-Titled by Moderator]

Original Title: Time Machine doing a full backup every time. My 237 GB data fills a 6 TB hard disk in less than a month.