Lack of "what" makes a cake tight or not spongy?

Scott has a point, I am a former chef and over mixing to is a key player in a heavier and more dense cake, separate the eggs and whip the whites to a medium h and fold them in the batter next time, bring the eggs to room temp for more lift, cream the er and sugar, even add a bit of milk for moisture, that can make a cake dense lack of moisture also.