[D] Why is the latent channel of stable diffusion so small?

Hi everyone. I'm a newcomer in generative AI, and I am currently diving into the inner structures of stable diffusion. I noticed that the VAE encode of SD uplifted the channels to 512 gradually, but suddenly drop to only 4 when generating the latent vectors. It's just like having a very very thin bottleneck after a very broad tunnel. Why is it designed like that? Is 64\*64\*4 really sufficient for expressing so much possible features.

channels diffusion encode generative machinelearning small stable diffusion vae vectors