Manipulating size as a transition trigger

I'm away from my desk at the moment, so haven't checked your .story file. But my experience has also been that state changes can be choppy in these circumstances.

I'd perhaps use two images and time the switch using the timelime, not triggers. So the first image only lasts 1 sec, to be replaced by the second image thereafter.

You could smooth this by using the new 'shrink' emphasis animation on the first image, and overlap the two images on the timeline ever so slightly, so it looks like the first image is shrinking to become the second image. A motion path could also make it appear it's shrinking towards a different part of the slide.

I used a similar technique in this demo (https://bit.ly/elhc464 ) to make the compass grow from small to large. Click the bottom right pocket.

It's two images. The first image has a grow emphasis animation, and when that completes, it's replaced by a much larger second image on the timeline.