Pinkie Pie was celebrating her engagement to Prince Blueblood. At the party, Big Mac ran into Pinkie's sister Limestone. Soon, the two normally reserved ponies were going at it, talking more than either had in months.
"Bucking apples is foal's play!"
"Nope. But I could crush rocks in my sleep."
"Oh yeah? Prove it!"
So he did. Twilight hypnotized him, and he still smashed boulders to bits.
Limestone was impressed with his powerful physique, and he with her humility. They exchanged many letters, and eventually married.
Ponies called the couple Apple Pie.