Frequently an error that makes cakes/cupcakes heavy is not whipping the butter and sugar long enough. If your recipe had you dump all ingredients together (I have a few of these and they always produce dense/heavy cakes) then rework the recipe so that you are first whipping the butter and sugar for a very long time (think 8 to 10 minutes -- just walk away from the mixer and do something else), then adding -- alternately -- your dry and wet ingredients (beginning and ending with the dry). This is kitchen chemistry at work. Your leaveners cause the batter to rise when cooking by expanding bubbles in the batter -- so, to get the lightest possible cake you want lots of bubbles. The best way to get bubbles is to whip the __ out of the butter/sugar combo. Also, make sure you are using cake flour (not all purpose) it is lighter and will give you a better result/lighter crumb. And if you did all these things -- well, letting us look at the recipe might provide a clue...

What is the recipe that you used? I'm not sure exactly what you mean by heavy? I think we'll be able to give you a better answer if we understand more about the situation - could you explain a bit more?