Yeah, I never really noticed before (I don't use curved rectangles very much apparently), but the curvature is a ratio and not a unit like pixels or whatnot. In fact Xara warns you about that if you try to type something like "12px" into the box. So when the scale of the object changes, so does the apparent curvature that's applied. On the other hand if you start off with shapes of those sizes and apply curvature, you end up with the same effect. On the bright side, at least this technique stops it from looking elliptical.