You take her advice.

"Yeah, it hurts, I can use a break."
You lean back and lay your head on her ample watermelon-sized breasts.
"You are quite out of place down here," Becca says as she rubs your displaced butt cheeks, "But it looks good, like me, but smaller. And then of course you get a nice big bulge in the front to show how much is in you. It does become pliable once it needs to turn, so it's not just going to bulge out like a stick."
"That's why the bulge is wider than me, because you are," you observe.
"Yes, that's right. I am a wide lady. This biggest head is about three times my size. You are going to have fun with that, I bet!" You put your hands on your cheeks as you think of it.
"I will have fun with all of them in at once, too. I bet I can do it some day."
"Naturally! Let's get this next cock in you, ok?"
"I don't feel stretched anymore, so I think that I need a bigger one."
"Right, you need to keep that ass stretched. So let me help you stand," she slides you off of her.
You stretch a bit from head to toe. You pat your butt after you finish. Becca hasn't moved. She waits for you to hop on the newly erect cock. So without further ado, you hop up and sit on it. You spread your cheeks and pull your legs back. While it is only your stretched and gaping hole against it, you are still just sitting on it.
"It's a little big, can you pull me open?"
"Of course," she says as she gets a grip on the edges of your hole. She pulls you open, and you get that stretching feeling again.
Then, sooner than you prefer, it's over. You sit hard on her crotch, feeling empty.
"That's it? I need more," you moan. You grab at the biggest one, cramming it against your hole futilely.