Because we can see at accurate size when in Design mode, the Play button is mainly for testing the prototype and making sure everything works. Therefore it's important to be able to see everything... so large designs get scaled to fit.
When sending a prototype to people (via sharing) in full screen mode, I think the mindset was that it's important that they see it accurately (to judge type size, etc.) so designs are not scaled. For shared prototypes that are not in full screen mode, designs are scaled. It could be useful to have the option to determine how it works in fullscreen though. You could submit that feature request at https://adobexd.uservoice.com/ (do a search to see if it exists first, and if it does then upvote that idea).