Well, I watched a 2 hour movie streamed from Netflix the other day and my phone got a bit warm but not overly so, it didn't seem too bad. YMMV of course, do you happen to have a TPU case on it? I had issues with other devices overheating with those silicone & tpu cases which is why I only go hard cases that don't retain heat as much now.
Samsung Galaxy Note 10.1
Moto G 16GB US GSM