I have attached a sample which highlights an issue we have seen with Fit To Page.
In this example, zoom in to one of the circles. Then click the Fit To Page button. You'll notice that it works fine. Then click the Fit To Page button again. You'll notice that the diagram zooms out further, so the diagram has no longer been fitted to the page - there's too much space around the content.
Alternatively if you zoom out and then click Fit To Page, the diagram doesn't fit properly. It always has too much space around the content.