This has to do with the aspect ratio of your images. Slick always maintains the original aspect ratio of the image.
The height of each gallery is determined by the height of the tallest image in it. The width of the gallery is always the full width of the post.
The images you add are slotted in and centred both vertically and horizontally in the gallery. This prevents the page content from moving around when you navigate between images of different heights and keeps the navigation buttons fixed in one place.