It's standard for a device that is small in comparison.
Remember, screen-size should be compared to the device's overall dimensions. Far too many people assume that the screen size reflects the device's overall dimensions when in fact, the size of the bezel needs to be taken into account.
The truth is, a 3000mAh battery is just too small for the U Ultra's dimensions.
The U measures 6.39 inches on height, 3.14 inches on width and 0.31 inches on depth (that's 162.4mm on height, 79.8mm on width and 8mm on depth if you're eager to know how it's like with the metric system). The similarly sized Huawei Mate 9 measures 6.18 inches on height, 3.11 inches on width and 0.31 inches on depth (that's 156.9mm on height, 78.9mm on width and 7.9mm on depth in the metric system).
Notice that the Mate is slightly smaller overall, and yet it packs a humongous 4000mAh battery.
3000mAh is large on its own, but in the overall size of the U Ultra, it's rather petite. You might get solid results out of it, but once you use a phone like the Mate 9, you start to wonder how HTC thought that 3000mAh is fine for a device that huge. It's even more damning when you know that there's no headphone jack and HTC didn't even bother to make full use of whatever space it frees up.
Also note that it has to power not just a 5.7" QHD display but a secondary 0.2" ticker display as well. That's a whole 5.9" of display space to power. And since IPS LCD hasn't caught on much in terms of display efficiency compared to OLED, it's just not as efficient as its OLED counterparts.
I'm happy that you get solid runtime out of your U Ultra, but I can't and won't let HTC off the hook for packing a rather anemic battery for such a huge phone.