DIAGNOSIS It's normal for it to be warm, but "really hot" is subjective. These devices have no fan and dissipate heat through the metal backplate, so feeling warmth is expected during use or charging.
FIX First, check its environment. Is it mounted in direct sunlight? Is it tucked into a recessed outlet with no air space behind it? Is the included power adapter plugged directly into an outlet, not a power strip? Ensure there's at least an inch of air space around the back and sides for ventilation. If it's in a hot spot, remounting it is the fix.
IF THAT DIDN'T WORK The heat could be from a software loop. Check if the screen is staying at full brightness indefinitely. Go to Settings > Display and make sure Adaptive Brightness is on and the schedule is set. A screen stuck at 100% brightness generates significant heat. A hard restart (mic off + volume down for 20 sec) can also kill any errant process causing high CPU usage.
HEADS UP If the device becomes too hot to keep your hand on comfortably, or if the screen starts showing discoloration or artifacts, unplug it immediately. That level of heat can damage the internal battery and is a safety hazard. Contact Amazon support in that case.