I've noticed that for most mechs the heat capacity is related more to their size then their role. While that makes sense (big mechs carry bigger guns thus need more heat capacity), I would expect that mechs within the same weight class would have different heat capacities based on role.

Since the Supernova is the only all energy assault mech, I would expect it to have the highest heat capacity of all the assault mechs. Whereas ballistic based mechs would have so the lowest heat capacities.

Mech - Heat Capacity
Supernova - 61
Mad Cat 2 - 65
Blood Asp - 65
Daishi - 70
Annihilator - 56
Fafnir - 65
Demos - 69
Behe - 69
Behe 2 - 67

As you can see the Annihilator has a low heat capacity which make sense but why would a Fafnir or a Mad Cat Mk2 have more then a Supernova? The Demos is lighter then the Nova and is mostly ballistic (except for 2 med lasers) and it has higher heat capacity then the Nova.

The example above is just for assault mechs but I think it would be a good idea to look over all the mechs so that the values for heat capacity are set to something that makes more sense. For example a Crab (all energy weapons) at 50 tons has 38 heat capacity while a Shadow Cat (mix of weapons) at 45 tons has 42 heat capacity.

Other things also come into attention when balancing mechs. Energy based mechs generally have more free tonnage left after filling their weapons slots- leaving more tonnage left for heat sinks. iIRC the Supernova had a bit less capacity than the Deimos or MCII because we were concerned about it being abused.

There was a formula applied to calculate the head capacity for all mechs. It was not just guess work. You'll have to ask Jeho for specifics, but it depends largely on tonnage and slot types and amount of each slot type.

Oh, and balance concerns.

Matthew Ts explained it very well awhile back, backing it with principles of physics.