Time Machine twice as big as system data

The image you posted (above) seems to show about 160GB of space being used so I would expect your Time Machine backup to be at least that large, your other image shows 169 GB which is consistent, as some files are not visible but still take space.

The "Storage" tool that can be accessed in Settings or About This Mac is known to be inaccurate for some Mac configurations (for others it is accurate). The most reliable way to assess storage space taken and available is via Disk Utility.

I have seen three instances when large Time Machine backups take place every time: (1) Time Machine is being run once in a while only, maybe with ~ weeks in between backups, then backups are each very large; (2) a virtual machine is being run (say for Windows) and the entire image gets backed up with every backup; (3) the user has installed utilities (in one case it was Backup Loupe) to inspect or monitor Time Machine backups and they interfere resulting in large backups every time. For (1), just run backups more frequently, then each one is a very small incremental update, mine take maybe ~ seconds to run; for (2) exclude the virtual machine from Time Machine and back it up manually to a different target destination every so often; (3) uninstall "optimizer" type tools, these often don't handle MacOS updates well.

There are no Apple employees (including members of busy product development teams) participating in these public, user-supported communities. Thus, we are not Apple implementors nor can we address questions that expect that level of why directed answer.

Let's say you updated to Sequoia v15.5 but have not run Time Machine against a designated TM backup drive for awhile. It is still accumulating large snapshot files on your drive that when you do eventually perform the first Time Machine backup, will be added to the first full drive backup. This will rightly make it appear larger than what you expect. Even worse, would be several Gigabytes of third-party applications updated, or a Virtual Machine guest container on your startup drive.

I have never encrypted a Time Machine backup on macOS — ever.

Provided you have chosen a TM backup drive that is 2 - 3x the size of your startup drive, you should heed the advice already offered to just let Time Machine and the operating system do their thing and get on with your own productivity. That is the best answer are going to receive here.

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Not only that, I also don understand the varying data sizes in the storage inspector,

Nobody does. The displays are created with a Spotlight search, accumulating totals as the search progresses. As the category of each piece of data gets resolved, the totals for each category change until complete.

The categories shown are not Folders. Documents includes things outside of your Documents folder. Applications includes more than what is in the Applications folder. There are apps in other places, and the total size includes data created for the apps.

It is inconsistent and confusing, such that it is a waste of time to bother using.

Mr. Unko wrote:

I know that in the end that’s all I can do, but that doesn’t answer any of my questions.

“No need to think about“ is something I expected on Reddit.

I didn’t expect it to be too much to ask why something works a certain way and why the system is confused about documents sizes, not here…

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The long-standing recommendation is to have a Time Machine backup drive that is 2 - 3 times larger in capacity than the internal drive of your Mac. That avoids the hassle of potentially exhausting the available backup capacity and having to replace it with a new drive to continue backups.

All of my Macs have a 1 TB internal drive and a 2 TB Time Machine drive and I have never run out of Time Machine backup drive capacity. When I am done with a given operating system, and about to perform an upgrade to a newer major version of macOS, I label the TM drive with the current operating system, the last backup date/time, and set it aside. I start a new Time Machine drive for the new operating system.

The rule of thumb would say that a 1TB internal drive should have a 2-3TB Time Machine backup disk to allow for multiple generations of versions of changing files and transitory files.

Mr. Unko wrote:

I'm currently creating the first Time Machine backup on my MacBook and I don't understand why it backs up so much data that's not even present on the machine.

Not only that, I also don understand the varying data sizes in the storage inspector, it keeps saying I have around 80GB of documents the moment I open "Storage" (I unfortunately took the screenshot too late), which is then corrected to around 17GB.

Can anybody shed some light on why it keeps doing this and where this phantom storage comes from and goes to, and most importantly, why my backup is almost double the size of the system itself?

Thanks in advance

I would also have liked to chose Time Machine as as the topic, but I couldn't find it, sorry if this goes to the wrong community

Why fret over it. if there are other snapshots accumulated on the Mac prior to your backing up I could see this would be more than you would expect at first blush.

Run Time Machine and forget about— let the software work for you, I would not try to micro-manage /decipher the details of the complex database with a bit of proprietary magic sprinkled in.

You only interact with TM through the TM interface in the menu bar, not Finder.

ref: Back up your Mac with Time Machine - Apple Support