Bad horror-porn plot which is probably more porn than horror:
A mad scientist obsessed with gene-splicing and creating strange mutants to be exhibits in his freak show, and he decides to hunt down the protagonists to use them as test subjects.
He actually manages to convince one of the female protagonists to join him willingly by promising to give her the body she'd always wanted. He performs genetic surgery on her that gives her balloon 'superpowers'. The first thing she does with them is inflate her body into a ridiculously curvy shape by inflating her thighs, buttocks, and breasts.
She catches up to one of the other females of the group, who happens to be a lesbian. Said lesbian is half afraid, half insanely attracted to her former friend's new form. The balloon woman easily seduces the lesbian, then knocks her unconscious.
Later the other protagonists find the captured friend awake and tied to a chair with a hose sticking out of her rear, but they can't get to her because she's behind a pane of glass. The balloon woman appears and taunts the others. She demonstrates her powers by briefly turning on a gas canister attached to the other end of the hose, causing the captured friend's breasts and stomach to start inflating in front of the others. The balloon woman forces the group to complete a pointless task for her amusement, and warns them that every mistake will cause their friend to grow bigger, and that if they make too many mistakes she'll pop.
Naturally the group keep making mistakes, so the captured woman keeps inflating. Mainly it's her belly, breasts, buttocks, and limbs that inflate, making her look more like a particularly round BBW. The group complete the task before she pops, but by that point she's so swollen she can hardly walk. Her arms and legs are really thick, her cheeks are swollen, her breasts and rear are like melons, and her belly's like a space hopper or a yoga ball, so naturally it's sticking out of her shirt.
Of course, other characters would meet other transformative fates that conviniently coincide with other fetishes...