There is no extra space at the bottom, it’s just that your images aren’t the same size. Try and upload images that have the same width/height (doesn’t have to be exact, just close as possible) and then download and run a plugin called Regenerate Thumbnails https://wordpress.org/plugins/regenerate-thumbnails/ and that will do its best to resize the images to the size of you have in your settings as much as it can without distorting the image.
