A spell gone wrong causes Spike to grow when aroused. Twilight wounds up trapped under his sensitive balls.

“Ooh.” Spike’s tongue hung out of his mouth, his eyes caught in a dazed expression. “Twi, that feels good.”

“That’s odd.” Twilight stared at her spellbook. “I don’t remember it inducing physical pleasure.”

Spike just bit his lid and glanced down between his legs. At some point during the testing, his member and balls had emerged from his sheath. Both were slowly getting bigger - his shaft getting harder and longer, and his balls swelling up with cum. His claws hovered over them for a second - should he really do this now? - but then he decided that it really wasn’t anything that Twilight hadn’t seen before.

He shivered a little bit as one claw made contact with the fat, puffy head. A warm, electric tingle danced up his spine, but he passed it off as mere arousal. Little did he know how right he was.

Twilight, meanwhile, was still staring at her book. “So do you feel mentally enhanced at all? Like your mind’s been broadened? Or - “

She paused at the sudden moan, looked up, and paled. “Spike,” she said. “What are you doing?”

“Mmff - nothing, Twi,” Spike said. His right claw jerked up and down his twitching, throbbing shaft as his left fondled his swollen balls. Given Spike’s adolescence, though, it wasn’t that surprising to see him taking a moment to bust a quick nut. No; what concerned her was the way his head was gradually inching up into the air, his arms and legs lengthening with each stroke of his cock.

“Spike,” she said again, “I thought you told me that you double-checked the physical components of the spell!”

Spike, who’d gone from a mere five feet to a much more impressive six in the span of about twenty seconds, just huffed a little bit and rubbed his shaft a little harder. “I - unf - did. Really. Probably.”

“Great!” Twilight threw her hooves up. “So now I’ve got a growing, horny dragon in my basement, and my spell doesn’t work!”

Spike just grunted and sped up his own pace. Even as his head-spines hit six and a half feet in the air, his work on his cock was far from forgotten. He groaned and fidgeted in place as his shaft grew up past his hips and toward his belly button, his balls hanging heavy below his crotch as they swelled with seed.

Twilight took a step in front of Spike. “And do you really need to do this now?”

Spike just kind of shrugged. “It - unf - feels really good!” He shot up a few more inches, surpassing seven feet - and then kept on growing. The rest of his body grew too, and not just in height. His reedy arms were rapidly thickening with muscle; soon, his biceps were each the size of a basketball. His pecs tightened across his chest as well, starting to bulge and swell with power and size. Meanwhile, his shaft had almost reached his pecs, and as Twilight watched, Spike started to bend over a little bit, his mouth open wide and tongue protruding.

“I can’t believe this,” she said, to nopony in particular. “I can’t believe this! Four hours’ work, all down the - agh!”

Another growth spurt hit Spike, and it hit him hard. In the space of three seconds, he nearly doubled his height, going from seven feet to twelve so fast his legs barely had time to adjust. His head scraped against the ceiling of the basement and his cock surged up past his tongue and into his mouth. Spike let out a muffled cry of surprise, his much larger body losing its balance and toppling over -

- with his enormous, beanbag-chair-sized balls landing directly on the unfortunate Twilight.

Twilight just groaned as the two cum-filled globes gurgled and sloshed above her head, weighing her down to the floor so that she could barely move. Far above her, his head above any pony’s reach even sitting down, Spike happily sucked on his long, thick cock, streams of clear precum dripping down his chin.

And he kept growing. Twilight could feel the balls on top of her getting heavier and warmer. Their musk was soon all she could smell. “Great,” she groused, again to nopony in particular. “Just great.”