Hello there, @sbridge ...
That is an interesting observation that you've brought to light. I just tested this in my own course, and I can replicate what you describe. Unfortunately, I do not believe there is a "default" zoom view that you can set for all of the documents submitted to you by your students. If you wanted, you could submit this as a new Feature Idea here in the Canvas Community website. Just click on the red "Suggest an idea" button within the link that I have provided.
I know this probably wasn't the answer you were looking for, but I hope it has helped to answer your question. Take care...be well.
