This is a peculiarity in the way the Ultra Keyer works – you’ll notice that you don’t have the same issue with Primatte.
Here’s a workaround.
Instead of piping your Ultra Keyer output directly into the merge, use it to mask your foreground (green screen) image.
To do that add a Channel Booleans tool to your foreground image and then pipe the Ultra Keyer output into the foreground input of the Channel Booleans. Set the mode to Multiply and then set all the channels to take Alpha FG.
If you then take the output of the Channel Booleans and pipe that into your Merge, you will avoid the cropping issue.
There may be another way around it but that one works. You can use the Merge Size control to adjust the size of your foreground and can dispense with the Transform node. If you do keep the transform node, make sure that you use it to affect both the input to the Keyer and the input to the Channel Booleans for obvious reasons.
Hope that helps.
Simon Ubsdell
tokyo productions
hawaiki