Hi there.
I am not an expert and I not sure I understand exactly what you are referring to. But I think that the strange doddering of the image is due to the changing in the number of onscreen-pixels covered by one pixel in the art. It is an approximation problem: I may guess you are using the “nearest” filter to draw the pixel art on the screen, and the onscreen-pixels does not “spring” at the same frame to the next pixel in the art. (I hope I am being clear…)
And indeed I guess slow scale animation is very hard with pixel art.
Anyway, just my idea, I may be wrong.