Your wording.

"It can change shapes and sizes, you said," she responds to your confused look as you feel your skin stretch. Your flesh squeezes around your fingers as you grip your buttocks.
You are aware of her dick once again hard poking into your crack. You gyrate for it as you did before but it feels better with your growing mass. It feels more effective somehow. Before you know it, it feels like you have two basketballs instead of butt cheeks. You can really bounce that ass now.
"I thought so. You got a rather large dose."
"Wait, your cum does this?"
"More specifically the combination of cum and precum. Each by itself does nothing."
"Well, I know you said I should stick to thongs from now on. Is that so you can splash me whenever you want?"
"Oh no, we aren't done yet. You should find it impossible to fit this ass in any pants that aren't customized."
"Oh so you want to cum again?"
"You have coaxed another large batch of precum out of me as we have been talking. If you bend over slightly, I can fuck your ass crack."
You do as she suggests. You feel it slide along and then poke you in the lower back. Then down to between your legs with a long stroke and back up to poke you in the back. You are thrilled by the feeling as more precum spreads around. She then suddenly holds the head just sticking out of the top and sprays her cum like a sprinkler. You have to reach far to spread it around this time. You can feel the skin stretching again as your flesh swells before you finish. After she finishes cumming you get it away from your butt by sticking it inside. Despite her large girth, you find your butt cheeks can come apart and slap together around it if you move properly.
"That's good, you already know how to use it."
"It's so big. Maybe we should stop. I look silly."
"Here's the good part then. Taken internally, it will smooth any growth that happens. So if I cum up your ass like...so...it will widen your hips and thighs to better match. Of course if we want to grow those areas, it will need to be applied topically."
"I look like a girl."
"So?"
"Well I'm not."
"Oh, well it's all just details. Most people consider breasts to be a good indicator. Let's do that. Turn around and jack me over your chest."
"Fine."
You turn around and kneel in front of her large balls. You rub the first drips around and let the head rest against you as you do your best to jerk off this monster log. You are not getting nearly as wet up top as your butt was. You look up and can't even see her face over the swell of her bust. You must be bad at this, or she is out of juice.