She doesn't.

"No fucking way," Amy tells you. "This is my place, I get to watch everything."
You pull the tail out of your mouth. "All lubed up, Red. I'll need my butt back, everyone."
The girls pull their hands from you, and Red slides up to sit on your butt. She gingerly plunges her tail into you, and with a contented sigh, cums all over Amy.
"Master, I've wasted my cum," she tells you with a wavering voice.
"I'm soaked," Amy admits.
"And she gives me the spanking," you wonder aloud as she starts bouncing her ass off of yours.
"Wish my cock was bigger," Amy voices while staring at the ass action inches from her.
"It will be, don't worry," Blue assures her. "You should keep up the current growth rate until it's time."
"So why does Amy get a cock? And why am I growing breasts? Two demons can drink from two breasts, and one from a cock. Right?"
"We are demons. We defile things," Red turns her head and smiles at you.
"Really."
"There are more complicated reasons involving human evolution, souls, magic and all of that which makes things easier to do it this way. For Amy, one demon will eat her "soul" through her breasts, and another her "lust" through her cock. The "soul" is better, so your singular demon will want to create access to that. That's why you get breasts and she gets a cock," Blue finishes.
"And why so big, then?"
"Imagine being born an adult, never having eaten before. You would be hungry, too," Red adds then licks her lips.
"The rules are just different," Blue concludes. "Now, as for my master..."
Your butt is laying over Amy's lap. Red is sitting on your butt. Blue squeezes herself between Amy's rack and Red.
"Getting to be a tight fit," Red comments.
"Don't worry," Blue chides as she jockies for room. She spreads her bust around Amy's.
"You better get that big butt outta here," Red demands.
"You know you love it," Blue shoots back.
"I sure will." Blue finally gets between Red and Amy and plops her butt down in Red's lap.
"Owwww," Blue howls, having impaled her ass fully on Red's cock, which was properly aimed.
"I warned you," Red reminds her.
"You split my ass wide open! I'm bleeding all over, I can feel it!" Blue yells through clenched teeth.
"Oh, relax. Nobody here will mind the effects if they do get any in them."
"That is..." you prompt.
"The blue demon trait, fatter nipples. I wouldn't mind," Red says.
"You are already exposed," Blue grits out.
"Oh my titties are tingling, that's true." Red rubs her boobs. "And master, it's running into your butt."
"I can feel the tingling."
"Poor Blue, here let's put that cock between my boobs," Amy consoles her. "Just barely fits."
"Thanks, Mistress," Blue tells her.
"What's it like fucking such a big butt?"
"You mean two big butts?" Red asks.
"I'm not that big...am I?"
"Not gigantic, but big enough to be big," Amy confirms. "Like two hams with a thong between them."
"So, um, another question. Will we be this big forever? Do I get to keep my boobs?"
"Yes, you both will keep your extra bits, but without carrying a demon inside of you, you will shrink to a more normal size as most of the magic leaves you. That takes like a year." Red takes a deep breath. "But if you want us to re-impregnate you after birthing, you will maintain a suitable size."
"Suitable size?" You inquire further.
"With each consecutive pregnancy, you will both maintain or increase your large size, and retain more if you ever decide to stop. If you end up doing this for too many consecutive times, your body will be so infused with demon magic, that you will end up self-impregnating if you ever choose to stop being impregnated by us. Your dick will grow long enough to fuck your own ass, and you will be unable to resist dumping cum into yourself for long when not pregnant."
"At least you will have a killer bod," Amy chuckles.
"You too," you retort.
"One step at a time. First task is making sure you live through the first birthing," Red reminds you both. "My master is making excellent progress."
"I'm more looking forward to having a dick than taking one," Amy admits.
"You have two holes to stretch out, Mistress," Blue reminds her. "Even I am getting in on it, apparently."
"I will too," Red quips. "Amy can stuff her big dong in any of my holes."
"Oh baby, now that's what I like to hear."
"I have dibs, I thought," you tell Amy.
"You do! But some holes may not be comfortable."
"Like what?"
"Mouths, dick holes, nipple holes..."
"Mistress! That is very erotic!" Blue exclaims.
"You like that, huh? Look at those fat nipples. They are like a foot long and as thick as my thigh! They can't wait, can they?"
"I want to see," you can't see well with all of the ass and tit.
"This one is mine, do I need to remind you? If you wanted mass instead of shape, you should have ordered it." Amy sticks her tongue out at you, but you can't see it.
"Nah I have you for that, Amy."
"That's right. I'm gonna be big."
"Yes, my Mistress. We will be shooting cum at each other, and pumping it into each other before long."
"Wait, something else just crossed my mind. So if I end up getting essentially 'addicted' to being pregnant so bad that my cock grows long enough so I can do myself up the butt, what about Amy? She can just impregnate herself no problem from the start?"
"Well..." Blue begins, "You won't be able to get your butt pregnant, but you will be able to get yourself pregnant...shall we say...the traditional way. You can impregnate your pussy with a normal human child. And to be perfectly honest, you might end up doing that a few times with how you act."
"Really? Interesting. But wouldn't they end up retarded?" Amy asks.
"Not definately, Mistress, but all the normal risks apply. You can of course infuse it with demon magic at any time before birth to prevent that and produce a hybrid."
"Amy is lucky, so many options."
"Jealous?"
"Of course! Mighty cock, double the demons. What do I get to do? Just lay here and get fucked by all of the demons we will make?"
"Not just them! Me too! Pretty good gig, just relax and bend over whenever a cock needs you. Or more than one," Amy giggles.
"Master, it is not a bad deal. Being a birther is a satisfying life, especially when we can convert their allegiance to you."
"So after long enough, will Amy be able to knock me up too?"
"Well, yes. You will have a lot of choices, or maybe just choose them all in turn!" Red's tail in your ass is stretching you again as it swells. "I love my Master! Oooohhhh!"
"Ooooohhhh! You are cumming in my ass! Stop!" Blue tries to escape but can't.
"Oh. Oh, yea..." Red sighs as her tail starts to shrink. "You have the figure for it, stop complaining."
"Look how fat her nipples are, she likes it," Amy announces. "Don't you, my busty demoness?"
"Only because you do, Mistress. Should I return the favor?"
"Wait, don't tell me Red knocked you up?!?"
"I did. I also said that this was a bad idea, remember?"
"No, don't shoot cum in her. Use me if you need to. I want her to keep her shape," you tell them.
"Oh, fine," Amy relents. "You can take this fat cock up your butt, then. Go ahead, everyone untangle themselves."