The CPU on these can heat up if something is running constantly - e.g., if you're stuggling getting a good 3G/4G connection because of the area you're in, or have GPS turned ON and not getting a good connection. I run the WAZE app with GPS on from time to time and that sucker really gets my phone pretty warm compared to the NAV App. Streaming APPs can also cause it to heat up if you don't have a strong signal. Just for grins, try putting the phone into AIRPLANE mode when it gets hot ... does THAT in effect cool it back down? If so, I would suspect a connection issue going on.
I agree with Cody if the phone is struggling with a connection of the Tower or even Wi-Fi it can pull it Down to were it will heat up trying to stay with the connection) if there are any intensive Apps running at the Time it cause this as Well and since you have a battery that can't be removed or the Cover can't be Lifted away it's little harder to radiate the Heat) what i use on Occasion if one of my Maxx's are Getting a little to warm is one of those little Battery operated Fans you can use to Cool off with) some of those little fans can put off in a Concentrated area Quite a bit of Cool Air) but when i do use it i do it sparingly so a My Maxx doesn't get to cooled Down to fast to cause Damage the Glass..