Settings > Photos >Transfer to Mac or PC>Keep Originals - Question

In reference to this thread:

https://discussions.apple.com/thread/8351572

"While trying to import photos and video from your iPhone onto your PC, I kept getting an error message. I would go to This PC/AppleiPhone/DCIM and locate the media I wanted to import. Then I would attempt to copy and paste the media into a folder on the desktop. I got the error message "Device is unreachable." I was able to fix this problem by simply changing a setting on my iPhone. If you have the same problem, go to Settings/Photos and scroll down to "Transfer to Mac or PC." "Keep Originals" should be checked, not "Automatic." Simply changing this setting solved my problem."

Doing this has allowed me to copy my large video files to a hard drive using a PC - but I noticed that the file sizes have drastically reduced, and playback of some videos have lost a lot of frames (I have been recording in 4k and 1080 120 fps. Why does this change when changing the "Keep Originals" settings?