To give it a more scientific explanation, the average female superhero body style is largely unattainable by all but the most genetically blessed normal humans. Training, dieting, plastic surgery, and posture only get you so far and can become quite expensive. To attain the superheroine hourglass figure, cosplayers looked to history, specifically to the corset and its cousins, the bustier and waist cincher.
A proper corset comes in two varieties, an overbust and underbust. The difference is, as you may have guessed, where the top of the corset lies on the body. An overbust corset is used in place of a bra, while an underbust can be worn with a bra. The most important thing of note about a corset, however, is where the bottom sits. A true corset will actually flow down over the hips to fully accentuate the hourglass look by rounding out the hips while shrinking the waist. All proper corsets will have boning in them to fully hold the shape, will lace up, and is considered to be an undergarment. The health risks of wearing a corset are well documented. I personally think that only a few female cosplayers are dedicated enough to use a true corset, as it is very dangerous to wear especially if you plan to move around a lot and pose.
The bustier is very similar in purpose to a corset, but will use soft boning (if any at all) and will come to rest on or just below the waist. The purpose of it is to boost up a woman's natural curves and it is often found in many different fabrics, designs, and accents, including beading, sequins, and ribbons, meaning that depending on the bustier, it can be worn as an undergarment or an outer garment. This is what I believe most female cosplayers are wearing when they wear a "corset."
The waist cincher is pretty much what the name implies. It cinches the waist inwards for that little extra oomph without the dangers of a corset. Basically, it's a really wide belt.