All else being equal, fat bottom distortion means the whole drawing is too low down.

The polargraph drawing rasterisation system is weird. The pixels at the very top of the page are very narrow and tall diamonds. The pixels from the very bottom row of the page are wide and short diamonds. So the top row has 30 pixels across, whereas the bottom row only has 15 across.

When there's distortion like this, it's because the machine is trying to draw 20 pixels at a point on the page that only has room for 15 pixels (or so). (Alternatively, the bottom row could be correct and all the rows above could have too few pixels in.)

I get my mental model of the "flow" of pixels in the picture from looking at the image in the controller and changing the size of the grid, and seeing how the squares move through the grid - they come in on a curve from the bottom corners and swirl up into straighter lines as they move up. That gives me some feeling for what would happen to a drawn set of pixels if they were pushed up or down on the page - what would happen to the overall shape of the drawing.

sn

i think ive figured out the problem- adjusted the mm/rev based on my sprocket size, which was 91 but then looked back over my adafruit receipt and saw the stepper motors were 200 steps per revolution. i had the controller set to 800 like guide said (even though I couldnt find motors the instructable used) so I scaled the controller down to 400 steps per rev and this resized my machine specifications- now the drawing that im doing now is coming out without distortion towards bottom, to size (like how its displayed in the controller), and the input picture has better proportions than usual (not as stretched out horizontally as it was). it looks amazing!