I found that kubelet is about 128MB big, kubectl is about 40MB and kubeadm is about 40M. However they are too big to run on IoT or some embedded system. Is there any method to reduce the size of Kubenetes?
I would check out k3s, which is kubernetes distribtion designed for IOT.
kubeadm is only one of the ways to set up a Kubernetes cluster, you can follow an approach that doesn't require it, such as manually setting up the services. For example Kubernetes The Hard Way doesn't use kubeadm - you can check it out.
kubectl will probably be necessary during the initial setup, but not afterwards, since it is only a client tool, does not necessarily need to be on the machine that runs kubelet.
You can also install kubernetes via a method you choose (kubeadm/manual/other), then delete all tools/packages/temp files that are not needed anymore, and prepare for example a disk image.
Also you should check out k3s, which aims exactly to reduce its size and trim everything except base functionality.