I don't run Windows 11 so I haven't taken a look, but I speculate it's because it contains a bunch of ML blobs for Windows Photo's image-classification and photo subject/contents keyword search.

On Windows 10, the Photos app package is about ~140MB on my computer. A good chunk of that is because the package includes a lot of dependencies - including platform deps that I'd expect would be part of the UWP runtime in the OS - kinda like how since the introduction of Swift/UIKit/etc in iOS the IPA packages all bundle their platform dependencies, even though they're demonstrably redundant, because UIKit isn't an OS-provided framework anymore... I'm not up-to-date in the iOS dev scene so I'm unsure why Apple went with that approach.