I have a Zenfone 2. First hand experience here. The weakest in the line (dual core, 1.6GHz) already runs pretty hot and is very unpleasant to touch when playing graphically intense games. It is smaller than the other models (5" vs. 5.5"), so cooling is probably not as effective, but the build material and design is pretty much identical otherwise. So I imagine the other models have the same problem.
And to your second comment, more circuitry = more heat. More cores = more heat. Higher frequency = more heat. Everything leads to overheating. It's already "edgy" with the phone I'm using - it's fine, but could be better - so just imagine slapping a Surface 3 -grade processor into it.
Battery life could be fixed by making it thinner, but cooling is a huge problem here. Sure, you can do something. You can make it have a full-metal body. You can then give it a more powerful Atom. But it will still be underpowered compared to Core i3-5005U, for example. If you look at devices like the Ockel mini PC, you can see that they aren't exactly phone-sized even if they have similar size screen. They're thick, and their shape is weird in a way. It has to be weird. It has to act as a heatsink, the more surface area the better.