(Anthro) Thunderlane accidentally gives Rumble a magic amulet that makes Rumble a foot taller whenever he OR Thunderlane cums. Now Rumble won't stop jacking off and Thunderlane has to fight not to get hard himself from his bigger brother's musk.

“Yo, Rumble! I’m ho - “

Thunderlane froze midsentence when he saw who was waiting for him in the living room. Admittedly, it was less who was waiting for him then what they looked like.

Rumble raised a hand from his place sprawled out on the couch and waved. The first thing that Thunderlane noticed was that his little brother was completely naked. The second thing he noticed was that he looked somehow…bigger than this morning. “Yo.”

“Uh.” Thunderlane licked his lips. “Dude. Mind telling me why you’re jacking off in the middle of the room?”

Rumble just snickered and stood up. To Thunderlane’s dawning horror, he realized that his little brother - who just this morning had barely come up to his chest - was now looking him straight in the eye.

“Dunno,” Rumble said. “I just know that it feels really good.” His hand went around his erect cock and started pumping.

“Rumble,” Thunderlane said. “Don’t make this weird.”

“Mm,” Rumble said. “Like, really good. And after the first time I jerked off this morning, I suddenly felt so much bigger. So much stronger. And then I realized that none of my clothes were fitting me anymore.”

“Rumble,” Thunder said. He swallowed, noting how into it Rumble seemed to be getting. “Cut it out, man.”

“I’m just as tall as you, bro.” Rumble grunted, his hips jerking slightly as he rubbed one out right in the middle of the room. “Not so little now, huh?” He grinned down at his generously sized cock, which had to be at least eight inches long. Thunderlane shifted uncomfortably. He was only seven inches hard. He should have felt embarassed - but why was his crotch suddenly feeling so tight? It couldn’t be…that he was getting turned on by this? “And somehow, I don’t think that’s the last of it.”

Thunderlane suddenly noticed the gold stone dangling from Rumble’s neck glowing faintly. His eyes widened. That thing had been legit? No way.

Fuck.

“Rumble,” he said. He held up his hands, palms out. “Let’s just think about this.”

“Oh, I’m thinking about it. And it just feels so good to think about - being bigger, so big you have to look up at me all the time, so big that I barely fit in the house.” Rumble was starting to pant a little bit now, his eyes closing as he lost himself in fantasy. “The biggest stallion in Equestria.”

“Rumble - “ Thunderlane began, but then Rumble was crying out, his hips jerking erratically as rope after rope of thick, white cum blasted from the tip. For an instant, he found himself jealous of Rumble’s load - he’d never cum that much - and then found that jealousy replaced by dawning horror as Rumble grunted…and started to grow.

Thunder stumbled back a few steps as his brother grunted and panted, his whole body bulging somehow. His head inched up - slowly at first, but then shooting up into the air. Thunderlane watched in a mixture of awe, dread, and arousal as his “little” brother added inch after inch of height, pound after pound of muscle, and - of course- even more length and girth to his already impressive stallionhood. By the end of his growth spurt, Rumble had to be at least seven feet tall. Thunderlane barely came up to his chest!

“Hey there,” Rumble said. He leered over his bulging pecs as his erection twitched and dribbled a bit of pre, already hard again. “Little bro.”

Thunderlane’s mouth went dry. No. No, no, no. This couldn’t be. This could not be happening. His brother couldn’t be so much bigger than him - couldn’t be so much stronger - couldn’t be so much more of a man than he was. And yet there, throbbing slightly and almost a foot in length, was his brother’s stallionhood, more than ready to prove him wrong. This was happening, and judging by the sudden moistness in his crotch, at least some part of him was more than willing to accept it.

He looked up at his “little” brother and smiled timidly. “Uh,” he said, his hands crossed over his crotch to hide his boner. “Hi there.”