Make them big

Pinkie's dick is 16 inches long and 4 inches thick when hard, her balls are the size of big oranges and are extra virile.

Sweetie's dick is 12 inches long and 2 inches thick when hard, her balls are the size of big lemons and are extra productive.

Fluttershy's dick is 14 inches long and 5 inches thick when hard, her balls are the size of small mellons and are normal in production.

Twilight's dick is 10 inches long and 3 inches thick when hard, her balls are the size of small mellons and are ridiculously productive.