You pick up the training bra. It's larger than the others, has no straps and is made of a very funny feeling material, most likely completely synthetic. You decide to try it on just for fun even though you know it won't fit you, and after taking off your torso clothing you slip the bra on over your head. As you expected, it's just hanging...
You suddenly feel a tingle in your nipples, itching as if they're aflame, as if they're inflating. After a few seconds you start feeling a bit of warm flesh against your torso as you slowly see the bra inflate into two big lumps, a big round hill beneath its texture that stretches the bra out so far that it fits tight. You look straight down and you can barely see your own stomach anymore due to your elevated chest, and when you move it wobbles a bit. You slide your hand across your new globes, feeling a twitch you've never had before as soon as you move your hand over a nipple. Not only did you suddenly grow fair-sized breasts, your nipples have also changed and become a lot more sensitive.
They are not huge, but the medium-sized breasts have a perky tightness that's almost enjoyable. The only problem is that you'll most likely be wearing this thing for a long time to come.