The second sentence seems strange, because tell is not used in the sense of speaking up here. If you do use it literally, you could see a sentence like
Your friend could tell you more about your girlfriend than you know.
Which means that if your friend would tell you something about your girlfriend, he would tell you things you didn't know about her.
Your chin doesn't talk. But it can tell you something - maybe because of its shape. If it tells you something, it is doing so the whole time; your chin doesn't sit down for a chat with you, it conveys any message it has continuously.
We all have them, but your chin could be telling you more than you know about you and everybody else.
So this sentence means that possibly, your chin is conveying a message about you (and other people?) that you didn't know about.