Movie clip resolution changes when added to an input node in Compositor I'm using Blender as a video editor.

I want to remove the green screen background from my clip and replace it with my own image. There are plenty of tutorials on how to do that, but I'm hitting a snag almost before I start.

In the Compositor, I created an input (Movie clip) node and a viewer node, and connected them.

When I put my clip into the input node, the resolution changes. The face of the subject seems to be compressed horizontally, and the clip looks to be about 5:4 or 4:3 aspect ratio.

Resolution of my movie clip is 1920x1080, and that is the desired output resolution. 

In Blender Properties tab, Dimensions are set to 1920 px x 1080 px (default).
Aspect ratio is 1:1

Any help would be much appreciated!

Thanks

Edit 1:



Yes the image I will be compositing is 1920x1080, however the error occurs before I even connect the background image. 

Edit 2: This is the second video I've made with Blender and the first had no issues like this (using the same camera) so I don't think it's a camera issue.
I used a Scale Node > Render Size as suggested by 3pointedit and it worked, still not sure how the problem was caused as I used this same camera before and had no issue. Thanks for the help though everyone!