Cake Flour and the amount of baking powder could help; however, this is my concern.
In the bowl of an electric mixer fitted with the paddle attachment, cream butter on medium-high speed, until light and fluffy. Gradually add sugar and continue to beat until well combined and fluffy. Reduce the mixer speed to medium and slowly add egg and egg whites until just blended.
The eggs are where you get your volume and light texture. Many recipes which ask for eggs and egg whites have you beat them first until very thick, light in colour and "fluffy" or frothy. The strawberries will weigh the batter down so only barely mixing in the eggs does not provide any lightness to the batter. I have a strawberry cupcake recipe I will see if I can dig out and compare it.