Do all of the nodes occupy the same size in the cluster?
No, all the nodes don't occupy the same size in the cluster. The Kubernetes components, such as kubelet, take up resources on your nodes, and you still need more capacity for the node to do any task. In the larger cluster, it is always preferred to create a mix of different instance sizes. So, the Kubernetes can easily schedule the pods that require a lot of memory with intensive compute workloads on large nodes, and smaller nodes can handle smaller pods.
