Alpha Channel weirdness

Weird thing
2 layers
Top layer - has a alpha channel based on colour key in Color tab
Bottom layer - is a PNG (but also tried with a JPEG)
When I zoom (either in Edit properties or Zoom in Color sizing) the top layer down below 100% it reduces the frame of the entire comp...
In Nuke this was cause the top layer ref image size was linked somehow to the bottom layer....
Any idea what is going on?
thanks
2 layers
Top layer - has a alpha channel based on colour key in Color tab
Bottom layer - is a PNG (but also tried with a JPEG)
When I zoom (either in Edit properties or Zoom in Color sizing) the top layer down below 100% it reduces the frame of the entire comp...
In Nuke this was cause the top layer ref image size was linked somehow to the bottom layer....
Any idea what is going on?
thanks