“greatly, yes, greatly.” These are the same words God has spoken in Genesis 17:2, so God is reconfirming what He said there, and retaining the emphasis in that verse. God promised Abraham that his seed would be a great multitude on a number of different occasions (Gen. 12:2; 13:16; 15:5; 16:10 (via Hagar); Gen. 17:6; 22:17).
“kings will come from you.” If nations are going to come from Abraham, then it is only logical that kings would come from him. Otherwise it would not be that nations would come from him, but only people who live in nations. For nations to come from him, then the leaders of those nations would come from him too.