Okay! So yesterday afternoon I had a really good call with the second level Apple support about this. He went into it in depth, and consulted the technical info that they have in regards to iOS 15.
First an update as to my own situation. The library on the 13 Pro Max is still large (100Gb) but now the library size on the XS Max has also grown to 46Gb. The overall space available has changed very little however. Basically, before the yellow photos block 'grew' the 'System Data' block was larger. All of a sudden (actually whilst I was on the phone to support), the photos block are to 46Gb and the system data block basically disappeared (more or less), suggesting that the system block was actually holding the photos library data that was being reoptimized at that time.
After consultation with documentation by the support guy, it seems that iOS 15 makes use of large amounts of free space to re-optimize the photos library to improve the local experience by having bigger local images. Rather than simply keeping the smallest optimised images/videos locally, it does some sort of usage analysis/assumptions and downloads larger, or even original size, images and videos. This is how MacOS currently works, but with MacOS, it's shown as a separate 'purgeable' amount of data in the storage report. This purgable data will get automatically deleted as space becomes filled up. In a nutshell, it seems that iOS now does the same, but simply doesn't report it like MacOS. Also, iOS 15 is the first to proactively make use of the space to increase a photo library size like this. Previous iOS versions only downloaded the bigger images when they were accessed, so if you didn't constantly look through all your photos and view them at zoomed size, then the library size wouldn't change much.
This happens transparently. Also, as space starts to fill up with less 'purgable' stuff, the purgable data gets removed to free space for it.
The upshot is that in certain cases our devices may show more full than we expect. The take-away is that effectively it's another case where Apple are taking over the management of the free space on our devices to provide a better experience, but at the cos of that warm fuzzy feeling that comes with having a nice wedge of empty space on the device.
This is one of those things that polarises people. I must admit, I don't really care how it's done as long as it works, and as long as there's an explanation for the lack of empty space.
He's going back to the higher-level engineering resources (those that sit behind the wizard's curtain I'm guessing
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), and will come back to me in about four days with a final opinion.
Myself? Like I said, I'm kind of not bothered as long as it works. There have been many occasions where getting the best experience has meant giving up granular control over certain things, but storage is something that makes more people jittery than most other things I know... Sometimes, as long as it works, you have to ask yourself whether you actually
need that granular control. In my opinion, it's one less thing to think about. This is why I choose iOS over Android - I'm simply not that bothered about having the granular control in most cases -
as long as it works. IN this case, time will tell...