Why do most GAN (Generative Adversarial Network) implementations have symmetric discriminator and generator architectures?
The main reason is that you want the Generator and the Discriminator to be equally powerful. The base intuition of GANs is that the two Networks can improve themselves through competition. If one is way better than the other, they will get stuck into some unwanted equilibrium in which one beats the other all, or almost all the times.

For this reason, they are usually chosen symmetric. It's the simplest and most effective way to make sure their competition will remain balanced.