"Yes!" Arro hissed, "Big at last!"
Arro trotted over to where he had left his neatly-folded clothing. He rummaged about a bit, and pulled out a flask that was familiar to Mike - it was the second potion the farmer had bought from the alchemist.
Still grinning, Arro removed the flask's cork. "I'll be bigger yet, centaur. I may even be bigger than you, when I'm through." Mike remained silent, although his expression was one of obvious surprise.
Arro lifted the flask to his lips, and drank the entire contents. He then tossed it aside, and his grin becoming even wider, with a hint of frenzy in his eyes. "I can feel it!" he exulted.
Arro's entire body seemed to throb and pulse, as his muscles grew, becoming larger, harder, and stronger. His already burly centaur physique grew to mythic proportions, and his entire frame enlarged, as did his oversized horsecock. "Oh YES!!!" he roared, lifting his enormous arms into the air, and flexing his giant biceps. Arro was significantly bigger than Mike, who was himself impressively large. But Arro was bigger than any centaur Mike had ever seen, truly a musclefreak of a horse-man.