Two possibilities:
The source videos have a sample aspect ratio or display aspect ratio other than square pixels that is causing the video to be squished.
Or, the Video Mode (timeline resolution) is something other than 1920x1080:
Two possibilities:
The source videos have a sample aspect ratio or display aspect ratio other than square pixels that is causing the video to be squished.
Or, the Video Mode (timeline resolution) is something other than 1920x1080: