Maybe I'm just not doing it right, so... could someone try to use Screen->Ellipse(), and see if the ellipse generated is the right size?
[ZScript] Something up with ellipses?
Could well be; draw_primitives has been buggy in the past, and is very under-tested.
The ellipse was using the wrong y-position for its center; I'm not sure if that's the only thing wrong that you noticed, but it's fixed in the next build.