Hi Frank,
I did exactly the same, with a bigger target image my model is still stable but it starts foating above the target image. It could be that the model size is calibrated acoording to the target image size. And perhaps when the target image is smaller, the smartphone is closer by when initializing the space, hence more exact?
I am going to do some extra experiments today, if I find out something I’ll post it here.
After doing the same, going from a target image of 11cm to 180cm and changing the target scale to half the height (=0.090) it works very well for me. The stability might be even slightly better than with the smaller target. So not sure what went wrong in your case…