Why is cake dense and cupcakes are not

Hi. After reading other posts about adding pudding mix and an extra egg to the DH box cake mix and using baking strips, my cake came out exactly how I wanted it to. Moist, dense and chocolatey delicious. However, i made the cupcakes the same way but omitting the baking strips, they came out too airy and light. Is this because of not using the baking strips?