Importing MP4 files into keynote stretches the initial image but then returns to normal size on playing video

I have been sent a series of MP4 videos on whatsapp. I have downloaded them and then want to drop them into a keynote presentation. Sadly when the preview images are input into a slide they are stretched, if I try and reduce the stretching to fit them in to a slide they then play in a compressed screen so look incredibly thin. Does anyone know why this is happening