Perspective/Size of a person in a tracked alpha video

Dear all,
We have filmed a person and embedded the alpha video in Zapworks. The person is tracked to a target image and it’s working well. The only thing that is mixed up is the perspective. When I move the camera above the person it gets larger and when I go down with the camera (e.g. on the level of the table where the tracking image is placed) the person appears to be smaller. It should be the other way round. How can I change this?