transferred videos only small size not full size.

Hi, I bought the ixpand drive to copy video filmed on iphone 10 to PC for editing.

However, when I transfer the video it only displays as small size, as in not filling up the space it needs to (sorry, very non-technical jargon here.)

However, if I upload the video from phone to Onedrive, then download to my computer, the video displays at the full-size. 

Is ixpand drive compressing the video somehow - bewildered.