File in Photos app and Files app shows different size when transferring files In iPhone, I copied a file from Photos app to Files app as mentioned below.

  Open the default Photos app on iPhone -&gt; 
  select a single file 
  (or multiple files by clicking the Select button at the top right corner) -&gt; 
  click the Share/Upload upward-pointing arrow icon -&gt; save to Files


Now, the size of the file(some name like IMG_0567.MOV) shown in the Files app on iPhone is different from the file size shown in Windows OS File Explorer when I connect my iPhone to a PC using USB cable. Why is that?

Edit 1:

Go to settings -&gt; Photos -&gt; "Transfer to MAC or PC section" has 2 options called "Automatic"  and "Keep Originals". When the option "Automatic" is selected in my iPhone(this was the default option) then the size of the video file is shown in iPhone Files app as 51.8 MB but if I connect the iPhone to PC then in Windows File explorer the file size is shown as 113 MB

Any have, when the "Automatic" option is selected on iPhone then I am unable to transfer videos from iPhone to PC using the USB cable due to the error the device is unreachable. So I changed this option to "Keep Originals" in iPhone, then I am able to transfer the videos from iPhone to PC. The size displayed in the Files app in iPhone and in the Windows file explorer(when the iPhone is connected to PC using USB cable) is also the same now (i.e 51.8 MB in this case)

When transferring photos then I still use the "Automatic" option, otherwise the image files are getting some extension like.HEIC, etc when I copy the images from iPhone to the PC.
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/