Lightburn axis distortion

Check for some sort of mechanical binding along the axis that runs short. With power off, does laser module move smoothly and effortlessly to all corners of the machine? If you’re only experiencing this at larger sizes the binding could be occurring at a specific local position.

Often times this would be caused by stepper motor calibration issues but xTool doesn’t allow for calibration changes so that’s not an option in any case. Also, since you’re saying this is only an issue at larger sizes I would suspect something mechanical. Look for some of the following:

frame is not square and gets increasingly harder to move in one direction
debris or something impeding movement at some point
wheels, rails, or bearings are malformed
belts are malformed or potentially stretched along some part
Just do an overall mechanical review to see if there’s anything that could be preventing free movement.

I can’t tell from the pictures but how much variation are you seeing in width vs height for the circle? If it’s within a millimeter it’s possible that it’s due to the asymmetric shape of the laser dot. You’d need to measure very accurately to determine that.

If you wanted to do so systematically you could run 2 tests:

create 5 or so evenly sized squares edge aligned in a row. Cut them out. Then measure the width of all 5 pieces while held tight together.
repeat above but do so in a column. Compare the measurement against the output of the first test.