I think by 'network size' the GP is referring to the network hashrate. For proof of work networks to function securely, the network as a whole must have a much

I think by 'network size' the GP is referring to the network hashrate. For proof of work networks to function securely, the network as a whole must have a much higher computation rate than an attacker. Obviously capabilities increase the whole time, so the bitcoin network needs to be able to adapt - to do this, it adjusts its 'difficulty' approximately every two weeks. The power usage corresponds directly to the difficulty level set by the network. If you add more miners, the difficulty will increase, and power consumption will go up, if miners leave, the difficulty will decrease and power consumption will go down.