Size vs. heat

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What one has to remember about heat is that electronics only get hot because they are never perfect conductors nor perfect insulators {though we can make nearer-perfect insulators than we can conductors}. A perfect conductor will never get hot, no matter how much current you put through it, because the voltage drop across it will be nil and power = voltage * current. Nor will a perfect insulator, because this time, the current through it will be nil.

CMOS is based around two transistors, a P-channel FET which goes conductive when the gate is driven low, and an N-channel FET which goes conductive when the gate is driven high. The P-FET is trying to pull the output high and the N-FET is trying to pull it low. Both the gates are joined together, and this is the input. This is a simple NOT gate.

For a NAND gate, where any input 0 will drive the output to a 1, we have several P-FETs in parallel trying to drive the output high, and so many N-FETs in series trying to drive the output low. Each P-FET gate joined to an N-FET gate is one input. When they are all high, all the N-FETs turn on allowing the output to go low; when any one is low, the chain of N-FETs is broken, one or more P-FETs turn on, and the output goes high. For a NOR gate, where any input 1 will drive the output to a 0, we put the Ns in parallel and the Ps in series. You can make AND gates from NAND+NOT, OR gates from NOR+NOT, and any other combination you like. In fact you really don't need both NAND and NOR, because you can make either one out of the other; but it turns out they're equally as easy to make as each other in CMOS {not like many other technologies}.

In an ideal world this would never dissipate any power, since the input cannot be high and low at the same time so only one of the transistors will ever be on. In practice what happens is that the gates act like capacitors which take a finite time to charge and discharge. They do not switch instantaneously from conductive to non-conductive. So one stops conducting while the other is starting to conduct, and for a brief instant while the inputs are changing state both transistors are conducting a little. It's not a dead short circuit of course, otherwise something would give way ..... hopefully a fuse.

Now every time something changes state, you get a little pulse of heat. Which is why fast processors need cooling. Additionally, to make sure that the logic gate output has changed state before the next clock pulse, you need to make the gate capacitances charge up quickly -- which means using a higher voltage than you could get away with at lower speeds. But 2x more volts means 2x more amps means 4x more watts.

Smaller transistors should have less gate capacitance, and so be capable of switching more quickly.