Even though millions of Android tablets are sold each year, and Chromebooks use many of the same applications through the Play Store, most of Google's apps are not optimized for large screens. Google Photos is one app that has always looked horrible on Android tablets, but that's finally changing.
Google appears to be testing an updated layout for large-screen devices in Google Photos. In both portrait and landscape orientations, the bottom tab bar is moved to the left side, with more controls than the simple three tabs (Photos, Search, and Library) visible on smartphones. It's certainly a better use of the available screen space.
Current tablet interface
New tablet interface (Credit: N4s7 on Reddit)
I don't have the updated design on any of my Chromebooks or tablets, so I'm unable to test if there are additional design changes beyond the changed navigation. The info screen for individual photos is especially bad on tablets, as it hides the photo and displays data in a single centered column.