You want her to expand in a physically impossible way

You've had some pretty drastic fetishes since long before you met Carrie, but even though she said she's down for anything... well, that had to exclude the physically impossible, right? And all of your deeper fetishes tend to only be possible in fantasy. Tits bigger than the rest of her body, so pregnant she's immobile, her ass being big enough that you can sit comfortably on top of it without falling off. Sexy as hell in pictures and stories, but not something a real girlfriend can do.

But now she's here, looking you in the eyes, licking your cum off of her lips, with a combination of love and deep, deep desire in her eyes. Begging to know what exactly it is that you want from her... If there was ever a time to tell her, it's now.

You sigh loudly. "Alright, but I don't think it's going to be possible"

She smiles brightly up at you. "Well, you won't know unless you tell me, will you?"

...She sounds way too confident in her ability to make your fantasy a reality, is she serious? Or just underestimating your level of perversion?

So... you tell her, tell her the one form of expansion you've gotten your rocks off to the most...