Why differences in apps/photos vs. core/preview (url and size)?

Perhaps someone can explain why the photo-app uses different mechanism and sizes than the preview/file app. Why must it use “remote.php” and why no preview image size in apps/photos?

apps/photos:
https://cloud.server.tld/remote.php/dav/files/user/path/IMG_XYZ.jpg
24px × 24px (Skaliert zu 851px × 638px)
size: 1.81 MB

core/preview
https://cloud.server.tld/core/preview?fileId=149355&x=1440&y=900&a=true
1365-1024.jpg (JPEG-Grafik, 1365 × 1024 Pixel) - Skaliert (74%)
size: 408 KB

I tested it with the standard Nextcloud picture “Library.jpg” again.
2 MB full size and 500 kb preview.

If i open it in apps/photos in the overall all picutres preview i get correctly 500 kb.
If i go to the subfolder “Images” then Nextcloud open it with 2 MB.
You can test it at https://try.nextcloud.com

I think the problem is loading with “remote.php” and not using “preview”.
I think only “preview” users preview size.

Bug or feature?