Whenever I bake cupcakes, they usually come out flat on top. I want to make them so that the top is big & rounded and has a similar appearance to a muffin. I think all I have to do is just put more batter in the cupcake holders but I wasn't sure. Am I right or is it more complicated than that?

More batter – fill them 2/3 full.
You are correct! More batter will give you the 'puffy' tops!