This is pretty simple. As the battery gets depleted... its stored energy is converted into heat. When the battery if fully depleted... all of the heat has been dissipated.
Comparing a 2013 to a 2012 laptop... The battery has less than 10% higher capacity... which means that over the operating life... only 10% extra total heat will be generated. Close enough to call it parity.
The operating life is approaching 2X as long... the time over which that heat is dissipated.
So... approximately the same capacity... operating over approximately twice the time... by definition... it can only be dissipating (on average) about 50% of the heat.
Derate that a bit... and call it 60% if you like... but there is no physical way that it cannot be running cooler over the life of a battery charge.
/Jim