Merge node not behaving properly

I'm new to Fusion and node-comps in general. I've tried a few tutorials but I've run into problems with the merge node.

I've tried an easy thing to do: Planar tracked a background, then pulled in an image to place it on the tracked bakground (so that it sticks to it).

The node structure is this: Two media inputs, the background hand held camera and the image.
Then I've tracked the background using planar tracker, created a PlanarTransform node that I've put after the MediaInput for the image.

So far, the image seem to be tracked correctly when looking at it free floating without the background.

But when I connect the PlanarTransform to a Merge node as a foreground layer with the background as the background layer, the tracked image is "amplified" so that all the tracked motions are intensified.

I don't know why the image, which follows the planar tracking, would change when connecting it to the merge node.

Here's an image showing the nodes. (The Transform node doesn't change the tracked motion behavior)

Is the image the same resolution as the footage?