I'm no where close to being an expert on how devices operate and what causes them to build up heat, but I have played a multitude of different games/apps on my device. What I think it boils down to is how much is going on, on the screen. In terms of effects, how in depth the actions are and the constant changing of environment.
We've all seen the pictures posted by Laura and FDS about how beautiful the new scenery is, but in global and with the current App Store versions, there's not a lot of demand for the device to always be doing something that requires a ton of effort to produce. Again, thats my lack of knowledge opinion.
So long flights have never caused my device to heat up like when I play clash of clans for a long time or the super detailed racing games that cause it to heat up. Of course this could all be device specific too.