Day before yesterday I visited the Apple Genius Bar and was assigned a Tier 2 support guy. I went for multiple reasons, not just the Time Machine issue but also the wide spread issue with Ventura and the quick closing of the Apple Store window when launched.
I posed the question about basic size needs: disk to backup and backup disk. He confirmed what I already knew that to sufficiently cover my working files and documents, a 1 gig drive is covered by a 1 gig drive. This setup will not cause the original question...
"Time Machine couldn’t complete the backup to “PORTA XX BackUp”
Time Machine was only able to complete 81% of the backup because there is insufficient free space on the backup disk."
That issue is being caused by the need for a firmware upgrade, which is included in one of the two Ventura upgrades that I cannot currently access due to my Apple Store. So I am now going leap past Sonoma to Sequoia and hope for the best.
Bear in mind, I do not back up the entire drive, just the working documents, which at this writing is about 800gigs, the rest of the drive is OS, apps, etc.
Regardless, no matter how big your backdrop drive is, at some point, older files will need to be dumped. Here's where personal working habits and technical setup comes into play – and choice.
My MacBook Pro is a mobile workspace, I share all working files via SyncThing to my rack mounted Mac Pro that has a drive in the region or 10 terabytes, all of that is then backed up to a 20 terabytes remote backup system, this is accomplished with SuperDuper.
Even that capacious amount of storage, will, at some point, dump older files. That is the nature of transitory backup, unless you can afford 'infinite size backup' which does not exist, you need to make some choices. Files that need to be archived longer than years and years, like business files, I offload to optical disks.
I can understand that if your only computer is the one you need to back up, then yes, double -maybe triple the size of the backup drive. That is not my condition, my MacBook Pro with Time Machine, can cover me for the last hour if I make a dumb mistake or for that mater, going back a year or more. If my MacBook Pro gets stolen or is taken out by a meteor that I survive, then, as I am triple redundant backed up, no worries except that I have to replace a fairly expensive device.
Do not confuse what you prefer or your unique conditions with what is technically required.
"I trust my practical experience more than what I read in Apple documentation which may brush-stroke over fine details. I don't read what I want to believe, but what I observe in practical usage". – VikingOSX.
You cited no technical sources, I cited the company that makes the hardware and software and has been saying the same thing for as long as Time Machine has been available.
"If you want a more in-depth technical explanation you can search this site for the boring details, but it won't change the answer: You need more capacity. 2 TB might be sufficient." – John Galt.
Perhaps those boring details are worth your investigation in order to answer technical questions beyond urban legend and personal bias.
Again, to reiterate, the issue I have with Time Machine throwing an error and refusing to complete backup is a firmware update need, it is not an issue with the size of my backup drive.
