That could explain the horizontal bulge, but your horizontal distortion and vertical distortion appear very equal. I'll agree with jeyurkon that it's a PS issue.
Here's mine. It has a vertical issue so I have it cranked up pretty far, and the boxes are a lot taller than they are wide, but the respective spacing is pretty equal.
You may notice the same tubular piece of cardboard in front of my set, I couldn't get it to fit either. But even without it, my horizontal sweep is pretty linear. I don't think the lack of it is affecting your sweep.
If I bring the vertical size down to where the boxes are square, the centering is really poor, the top 1/3 of the screen is dark and the bottom is still just off the screen. I think maybe the cardboard tube out may have something to do with that. Or a physical mounting problem with the yoke. But that's a problem for another day. Haven't really worked much on this one, other projects ahead of it.
Glad the knob was a good fit, and glad I could help.