The default magnification level is calculated as the ratio between the width of the full-size image and the width at which the image is presented. So, the bigger (in terms of width and height) the uploaded image, the bigger the magnification level.
In the pro version you can choose the magnification level, but that is not recommended, as it actually stretches the image in order to fit to the desired magnification level. This leads to a slightly blurry zoom. The option to choose the magnification level works well for websites that already have hundreds of small images uploaded and having a bit of a blurry zoom is more acceptable than the effort to re-upload all the images.