Reduce jittery of huge models

Hi, 

I am using Image target with extended tracking to lay huge virtual world on to it. The image tracking works perfectly fine but there is a considerable amount of jitter/ shakiness of the objects when there are more and huge. I tested with smaller objects and the jitter is negligible. I don't understand why it is different with different size objects. Can someone help me through what needs to be done in order to reduce the shakiness of the objects?

Thank you in advance