With 8.9 mm thickness, the company might have added a much larger capacity battery, but I guess that would be too much to ask from a budget smartphone. (phoneradar.com)
However, one of the recent trends in portable solar setups is bigger panels paired with larger capacity battery storage (often called solar generators), which can be a huge boon when working off - grid or traveling or lighting up a tiny house. (treehugger.com)
The Pixel XL, LG G6, Moto Z Force Droid, and Huawei Mate 9 all have larger capacity batteries in smaller form factors so I am a bit concerned about the battery life of the HTC U Ultra. (zdnet.com)