It's totally normal for devices to heat up under heavy usage.

Try these tips for improved performance: 
1. Exit other apps before playing the game. 
2. Make sure your device is fully charged, or at least 50% battery life.
3. Be certain you’ve installed any recent software updates.

If you’re running an old version of the game or software, go ahead and update it.

Playing a large-sized game for a prolonged period of time will force your device to work hard, making overheating difficult to avoid. 

Using the information provided has been proven to reduce those chances, so trying them out is definitely worthwhile.