I think this is just a bug in the watch OS; this has happened to me before as well. If you are concerned about it further, id recommend making an appointment at the apple Genius Bar. Cheers!
Check you accessibility settings in the watch app on your iPhone. Go to General-Accessability and see if zoom is enabled. If it is turn it off. I had that on and my watch face would enlarge at times when I would check notifications. It takes a 3finger double tap to reduce the screen to normal.