Encrypted data ideally is incompressible. Incompressibility is a hallmark of efficient cryptographic operations.

See the Wikipedia article on Kolmogorov complexity, which has a short section about compression. https://en.wikipedia.org/wiki/Kolmogorov_complexity#Compress...

Edit: One of my favorite concepts in the domain of compression is the pigeonhole principle, that states that for all compression algorithms, some outputs will be larger than the inputs.

Well designed encrypted payloads may be compressed, but the outputs should on average be larger than the inputs, rendering compression useless thus it is said to be "incompressible".

https://en.wikipedia.org/wiki/Pigeonhole_principle#Uses_and_...