90MB is the size of the kernel, our init binary (written in go), the 12 binaries we include by default, and various required config files and drivers https://www.siderolabs.com/blog/there-are-only-12-binaries-i...

It probably accounts for the k8s release unless they download it post-boot?

K8s binaries are downloaded separately. Specifically they’re downloaded and run inside containers on the host.