How can I make cupcakes lighter?
My best guess is you are probably over mixing the batter. When you over mix, gluten networks develop leading to a denser and chewier texture.

You should mix the batter until "just combined" which just means until you can no longer see the ingredient you just added.



http://www.cupcakeproject.com/2014/05/what-happens-when-you-overmix-cake-batter.html