This is actually typical behavour of smartphones. The processor heats up after heavy usage, the same thing happends to a desktop or laptop however they have fans that help cool them off, our phones dont. Theres really not much that can be done about it except to maybe give the phone a few minutes to cool off in between heavy usage or just deal with it. The phone wont heat up to a point that its dangerous or will do damage and if it did i believe that theres a failsafe that would turn the phone off it nothing to really worry about..