You balloon.

Once the cum is flowing out over your expansive rack, it starts swelling. It grows up the most as she promised, but to be fair, they didn't have far to go. Your chin rests nestled in your cleavage as promised.
"So? It's not that much of a difference, I know...but it's cool...right?" She pulls herself out from between your tits and gives your big tits a hug as she pulls you to your feet. She also jerks your nipples until you lactate, and gets herself all wet by spraying herself with your milk. "Spread 'em!" You do as she says, and she moves around back and unceremoniously starts sliding her arms up your butt. You look over your shoulder and watch as she shoves her head into you next. Her shoulders come after, and then her titties slow her down. Since your recent feat of flexibility, though, you correctly predict that she quickly pops both tits into you with nothing more than a pleasured grunt on your part. Things get even wider as her full belly slides into you in ever-increasing dimensions. You took it before though, and so simply reach back and pull your cheeks apart as she makes your own middle grow. Your only concern is that her belly hangs low because of its weight; the farthest jutting part of her front might align with her ass in the back, creating a serious bottleneck in your anal adventure.