Big Mac enjoys himself on the farm while every other Apple is away on business. Works out while working on the farm, pumps reps with haystacks, etc. Involves self-worship, sweat, muscle growth, hands-free-pants-on orgasm multiple times.

Big Mac eyed the pile of haystacks. “Welp,” he mumbled, to nopony in particular, “Looks like I need to get you to the South Field.”

But… He squinted at the sun. It wasn’t quite high enough in the sky to call it a morning yet, so it looked like he might have a bit of time to have a little fun. Before he knew it, Big Mac found his right hand roaming over the biceps of his left arm, feeling the bulge and tightness of the muscle beneath his fingers and squeezing every so often.

He chuckled and let his hand fall to his side. “Can’t say no to that.”

He glanced at the pile of haystacks again, and a thought struck him. They looked awfully heavy. In fact, there probably wasn’t a stallion in all of Ponyville that could easily lift a single one of them, let alone two.

With a jaunty grin on his face, Mac leaned down, tensed his arm, and lifted a pile of three haybales into the air.

“Nothin,” he said to himself, still smiling vacantly.

He was already starting to chub up a bit, his stallionhood swelling beneath his working jeans. Mac ran his free hand over his crotch, feeling the fabric tighten and stretch over his erection as his other hand hefted the 450-pound “weight” up and down a few times.

He loved this - the knowledge of just how big, just how strong he was. He could already feel himself leaking a bit through his jeans. Mac pulled his stroking hand away.

The rest of that pile of haybales was looking mighty appealing right about now. Mac smirked and let the three he was carrying in his right hand drop to the ground.

Seven bales of hay. Over a thousand pounds of mass. Big Mac just chuckled a little and rolled up his sleeves. Suddenly, the fabric felt so tight against his biceps, his jeans straining ever harder against his growing erection. Then, with a grunt, he reached down, scooped both piles of hay up, and hefted them back up.

He took a deep, shaky breath - but his arms didn’t give. They didn’t even shake. As a grin slowly spread across his face, he started to pump his arms up and down, watching the enormous bales of hay rise and fall with his chest.

Over five hundred pounds in each hand. He pulled his forearms in tight for the next rep, really flexing his biceps this time. They bulged up and up and up, larger and larger until they were even bigger than his head, and then he flexed even harder, lifting the bales as high above his head as they would go.

“Hnggh,” he grunted, sweat starting to run down his chest and forehead. This was real strength - a half a ton over his head, and he was only just starting to break a sweat. He could smell his own musk on the air, half sweat and half sex, and loved it more than anything.

Another rep. Down, then back up. Mac felt his arms flexing harder than they’d ever flexed before, yet could feel the task getting easier as he went. He imagined his muscles swelling even bigger with every rep, his biceps gaining a little more size every time he let them relax. He imagined his pecs slowly inching out before him, his legs growing thicker and thicker and longer too, until he’d grown so big that he could barely fit through a door vertically or horizontally.

Big Mac’s breathing grew more and more ragged, though not due to fatigue. His chubbing member had reached full length now, where it reached all the way down to his knee through his left pantleg. He could feel his balls churning between his legs, could feel the inevitable tension coming in his groin.

He grunted again, this time breaking into a moan as the pleasure mounted. Bigger and bigger and bigger, he’d get, his muscles swelling along with him until there wasn’t a pony in Equestria who could compete. And even right now, who could compete? With a stallion who could lift half a ton of hay over his head and still have the strength for more?

With a final grunt so loud it sounded like a roar, Big Mac lifted the haybales up to shoulder level - and held them there. Five seconds. Ten seconds. Fifteen seconds, and his muscles were just barely starting to twitch. Sweat covered his body in a thick, musky sheen, and the dirt beneath him was already starting to become a muddy mixture of precum and sweat, even as his leaking cock completely ruined his jeans.

Mac could almost swear that he could feel - no, see - his arms bulking up, his muscles getting bigger and bigger with each passing second. He was big - no, he was huge. He was the biggest stallion around, the most enormous stud in all of Ponyville. Any mare or stallion would kill to feel these muscles, this incredible size - and he could see himself giving it to them, straddling them with his huge, bulging body. He could see himself growing on top of them, dwarfing them in his never-ending quest for size. He could almost see the veins swelling and popping on his biceps, his shoulders growing wider around him, his neck thickening into steely, corded muscle -

This time, he did roar. With a guttural moan, he thrust both arms into the air far above his head - and the tension at the base of his dick finally reached its peak. Shot after shot of thick, ropy cum sprayed from his flaring head to fill his dripping pantleg, his balls emptying themselves through the fabric as his muscles bulged and dripped with sweat. Mac let his head roll back in pleasure, though whether it was from the orgasm or the total admission of his strength, he couldn’t even tell.

On and on he went, cumming for at least a good two minutes before it finally started to subside. His knees wobbled, his hips jerked, and his cock flared and throbbes again and again as it spilled litres of hot stallion seed onto the ground - but Mac’s arms never budged. It was only when the orgasm was finally over, his pants completely ruined and the dirt covered in white, that Mac finally let his sweat-covered arms drop to his sides, letting the bales of hay fall to the ground with a crash.

A minute later, once he’d caught his breath, he picked them back up and got to work.