The truth about corsets...wait, let me rephrase that...the truth about fashion in the past is visual perception. Corsets were not only part of life, but the way they work is not often thought of much. When lacing a corset, it pulls evenly. This means that corset actually pull you into a round shape. Everyone knows that, normally, the waist is sort of oval shaped, making you flat in the front and very small from the side. When pulled into a round shape, it makes you small in the front, but from the side it may make you appear larger. Sadly, I have no diagrams to describe how this works, but you get the idea.
It should be acknowledged by everyone that people were healthier and naturally smaller back then. I'm not going to say that everyone was skinny, because that is most certainly not true. Corsets are part of what make you appear smaller.
The clothes worn on top also play a huge part as to how we percieve them. Take this picture for example. I love this picture, because it throws people way off all the time on Pinterest.
To use the visual perception example: she wears large skirts, which makes the waist they sit on look very small. Large sleeves also contribute. But the trim is key: notice how the trim on the front makes her look wide at the shoulders, then tapers down into a tiny point. This draws attention to that point, thus creating a 'wow, look at that waist!' moment. In reality, you can see her waist is not actually a part of that point, but the trim makes it look so.
Another fact about corsets: you cannot eat very much while wearing one. Your stomach simply doesn't have space for 'extra', so you basically can only eat the amount that your body needs. I've had it happen; I'll try to eat the amount that I normally do while wearing a corset, and major stomach aches quickly ensue. That is basically the best diet in the world!
Now, this girl, I would say, is sort of 'normal' sized, or appears so because the trim does not 'point' as well as the last one, although in our standards, she is fairly small.
Now, this image (the one below) has quite a few theories, mainly why she is photographed from the back. I can't think of a reason why not to, with such a beautiful dress and hair. Several suggestions were in mourning, or post-mortem. After the post-mortem theory, one lady commented suggesting that maybe she died of 'small waist syndrome, or lack of oxygen'. I got a laugh, although computers do not convey the tone in which the original is speaking. This lady is also very small compared to modern standards, but I wouldn't say unusually so. Her skirts, very full worn over a hoop, lend the slendering effect I've already mentioned.