There are 6 gallery styles available in SmugMug but I'm not aware of a zoom out function.
If you want to see all of the photos in your gallery on one page look at either the collage or thumbnail styles. Note that you will still need to scroll if there are many photos in the gallery.
Help page showing gallery styles is at https://help.smugmug.com/change-the-look-and-feel-of-my-galleries-H1u4eePyNrM
Yes, there's no proper zoom available, but you could set up a smart gallery in thumbnail style that automatically contains everything in the normal gallery. Then you could add a button in HTML to your gallery page that would take the viewer to the thumbs gallery, which could contain another button to take you back. As Denise mentioned, if there are many pics, you might still need to scroll, but that's the way it should be--10,000 thumbs on a single screen would be useless.