Really good story. I wish the descriptions of the growth and size observations had been a bit more detailed. The story regularly indicates that she's constantly near some potential bursting point, but that concern stays mostly static throughout the story, save for the one moment near the end where she looks truly panicked, but even that is very brief. It might have helped if a series of descriptions that progressively describe her body losing elasticity, or showing additional but different signs of approaching bursting were provided to give a sense of rising tension and approaching potential doom. Something that lets the reader realize that she's passing different milestones on the way to bursting, and constantly wondering if there are any more milestones left for her to pass, or if she's really about to blow right then and there. Just something that constant ups the tension rather than keeping it at relatively the same level throughout.
Additionally, the size of Colette's body according to the measurements given made it real difficult to imagine how she could have squeezed through the house and doorways, as well as how she was oriented in her posture throughout the story. Some description about how the shape of her body had to change in order to accommodate a girl who was effectively twice as wide as most of the passages and entryways that she passed through would have helped to make it more believable.
I was also hoping there might be more detailed descriptions of how her womanhood had changed due to the inflation. There were some, but that's a huge area ripe for story detail that you could explore in the future. How big was her clit? Were her labia looking as if they'd burst too? Were vaginal juices pouring out of her everytime her sensitive skin got roughly touched? Was the air pressure making her inner vaginal passage shallower by pushing her cervix closer to her entrance, thus making her boyfriend feel even bigger inside of her when he entered her? Lots of opportunity here to deepen the description.
The dialogue was excellent. It really gave good insight in to Colette's personality, and her addiction. Would probably be good if the boyfriend had gone in to further detail about the struggle of temptation he faced whenever he looked at her.
I'm sure folks definitely want to see her bigger. I don't want to see her pop, but if you can find a way to describe her right on the verge of it, and losing her mind to pleasure as she becomes a fucktoy because of it, that would be awesome for a part IV. I don't need to see her outgrow the room per se, but definitely if she grows big enough to the point where her skin is stretched so thin that she starts becoming a bit transparent, or stretchmarks appears, or some sort of description that shows that having sex is seriously a dangerous thing for her that still happens despite the risk to her life, that would be fun.
