[Stromaluski]

Originally Posted by 27litres

I like your thinking, but one of the biggest factors for increasing disc size is heat dissipation.
You're right in tyres being a critical factor in how efficient your braking is, and the leverage action of larger discs and calipers, but the larger size disc also provides a critical benefit in heat dissipation.
You alluded to it a bit anyway, so I'm sure you're aware of most of this, but brakes are all about energy management.

When the car is at speed, it has a large amount of momentum which is Potential Energy.
In order to slow the car, you need to convert that energy into another form (conservation of energy).
Cars use brakes to convert all this Potential Energy into Kinetic Energy, in the form of heat.
That heat energy then needs to be added to the surrounding environment through equilibrium as efficiently as possible.

A measure of how effective the brakes are, is how effectively they can shed the heat, which is a measure of exposed surface area.
So:
Ventilated discs are better than solid discs.
Bigger discs are better than smaller discs.
Larger pads are better than smaller pads (within disc coverage limits), and larger/multi piston calipers allow for larger pads and better control.

Then there's the engineering compromises of unsprung mass, physical wheel size, metalurgy, cost etc

But as you said, none of this is any good if your tyres lose grip!

Agree with all of this.