Raytracing: why are the spheres in the image below appear stretched?
Shapes appearing stretched in the periphery is a consequence of perspective projection. The wider the field of view (FOV) is, the stronger the stretching effect gets.

To demonstrate the effect I wrote a quick example on ShaderToy: https://www.shadertoy.com/view/MltBW2

As you can see on the images below (corresponding to FOV of 40, 80 and 120; if I didn't make any mistake), this becomes especially noticeable with wide FOV, like 90° and above.