File in Photos app and Files app shows different size when transferring files
Solution 1:
Yes, this is a known issue in iPhone
The below article provides more details(about all the issues you mentioned be it different sizes, not being able to transfer video files, weird extensions, etc) so please have a read
https://www.guidingtech.com/fix-device-attached-to-the-system-is-not-functioning-error-iphone/