Max awoke to the feeling of his body being pounded into by his mother's large horse cock. His slime jiggled in time to her thrusts, as she pistoned in and out of him. Slaps resounded around the room as she spanked his gelatinous ass, grunting as she grew closer to climax. Max entwined his legs with her watermelon-sized balls, squeezing them between his thighs. It was enough to bring her to climax. She unloaded into him with a moan and gave his ass one final slap before she pulled her cock from his body.
Max looked down at himself, stuffed full of cum again. Except, he seemed a bit larger than before. He certainly retained his human form better than before. Evelyn raised an eyebrow, bringing her perfect, beautiful face into a twisted pout. "You grew during the night," she said dismissively. "You absorbed my cum like the good boy you are, so, I decided to reward you."
"Thank you, Mommy," Max murmured, rubbing an arm across his swollen body. He could already feel it beginning to be absorbed and converted into slime. It only took a few more minutes until the white glob of cum within him had disappeared entirely. Evelyn watched with hungry eyes as he got up off the bed. What he noticed first was that he was taller than before. Where he previously couldn't see to the top of his mother's thigh, he now was head-height with one of her huge abdominal muscles. That would put him at... 5'10, a far cry from the 4 feet he had before. Even the figure of his 9-year-old self had shifted, filling out again to a more muscular version of himself. Evelyn frowned at him, and Max felt himself shrink under her glare, his desire to please her forming a cold pit in his stomach as he realized he'd disappointed her.
"Unfortunately, you grew as well. But don't worry my love, I'll fix you so that you can please me again." She hefted one of her enormous breasts, heavy with milk. "I am very full, after all."
"Mommy, can I get to my computer first?" he asked timidly. She sighed huskily but waved him through. He wandered over to the What If machine. WHAT IF MY MOTHER HAD NO REFRACTORY PERIOD AND COULD BECOME HARD AT WILL? WHAT IF SHE PRODUCED TEN TIMES AS MUCH CUM AS A NORMAL HUMAN? WHAT IF SHE PRODUCED TEN TIMES AS MUCH MILK AS A NORMAL HUMAN?
He pressed enter and laid back on the bed, legs spread, waiting for his mother. Evelyn smiled wickedly, her pumpkin-sized balls draped over the bed as her cock began to stiffen. Her chest pressed into his as she began to fuck him again, and he took her nipples within his slime and began to suck. Milky white substance began spilling into his body from where her breasts were encompassed by his slimy body, and as she thrust into him, she gave herself her own titjob through his slime. While Max sucked her dry, he made sure to knead her soft flesh and tweak her nipples, sending her gasping from the pleasure. He kicked his feet against her giant nuts, squeezing her shaft with every ounce of slime he could muster. Evelyn leaned down and placed her lips over Max's and pulled him into a deep kiss. His slime filled her mouth as she dominated him with her tongue.
Max broke away. "Cum for me, momma! Give me all your seed! Make me your perfect little fucktoy!"
Evelyn gasped as her balls spasmed, shooting rope after rope into Max. The slime boy began to swell up as more and more cum filled his body. Evelyn was pushed back by the sheer size of her load that filled the room, pressing the giant Amazon against the wall even as she continued to fire spunk into Max. She broke through the wall with a twitch, spilling out onto the loungeroom floor as her climax finally ended. She was finally empty. But Max wasn't.
Filled with a potent mixture of his mother's cum and milk, he took up the entirety of his bedroom and spilled out from the door and the new hole in the wall his super-strong mother had created. Then, he watched as everything was absorbed. He grew larger still, flowing into other rooms and completely filling them, Evelyn caught in the tide of slime. He swept her out into the backyard where he stood in human form, a boy that managed to tower over his goddess of a mother.
Evelyn stared up in horror at her boy. Now twelve feet tall, he had gained a whole two feet on her and was somehow even wider. That wasn't how it was supposed to be at all! Then, just as suddenly as he grew, Max began to shrink.
With each beat of his mother's heart, he shot down a foot. His muscles pulsed larger, then retracted even smaller, pulling him down below his mother's height. His slime grew darker as his body contracted, squashing his huge form down into the 4-foot form of her 9-year-old son. Max knelt before Evelyn, bowing his head. "I'm sorry, momma. I grew too big. I disappointed you."
Evelyn looked down at him, a dark slime, immensely large yet keeping his form small just for her. She took his face in her hand and tilted it up until he met her dazzling blue eyes. "Max, what are you?"
"I am your obedient sex slave, Mommy Mistress. I am your fucktoy. You may do with me as you see fit."
"Very good. So tell me, fucktoy, how strong are you?"
"I have all the strength of my true form, Mother. I am dense, though, and can hold whichever form you want me to have, but I always have the same amount of slime and strength in my body." He slammed a fist down on the pavement, and a spiderweb of cracks formed on impact. "My mind, however, is empty. You are the only thing that fills it. Anything you tell me to think, I will think. You may give me whatever personality you see fit, Mother, but even if you tell me to betray you, I cannot. I am your slave above all else. This will be always true."
Evelyn tapped her chin. "So, if I told you to become a dog, and think like a dog, you would-" She cut herself off, as Max had already reformed into a dark green German Shepherd, each thump of his tail denting the concrete. "Turn back, fucktoy. I want you human again."
She hummed and stalked around Max where he knelt, her fingers playing gently across his skin. "Now, for your personality..."