The mass of the device acts as a heat sink therefore a larger device given all other things equal stays cooler.
You notice this with the processor throttling on the rMini and Air. The rMini processor throttles faster and more then the Air as tested by Anandtech. It won't be a surprise to see this with the 6 vs 6+.
This imo is a non issue and not just because I personally don't play games on my phone but I think its just going to be one of those things.