The butt plug starts growing

Samus was trying to get up off the plug, but it caught on rim of her foot wide gape and wouldn't come out. That's when she looked back and saw the neck of the toy was growing wider.

Once it reached a foot wide it could finally start stretching her again, and so it did, slowly pushing out her gaped asshole further, 12 inches, then 15, 20, it was wider than a yoga ball, and the head inside her ass was growing to match, now the size of 2 beanbag chairs bulging out her stomach.

Her mind was getting short-circuited by the pleasure, and her butthole only got wider.