WordPress Creates Unused (Unregistered) Image Sizes

By default, WordPress will automatically generate a few different sizes of images. You can adjust the sizes here: Dashboard -> Settings -> Media -> Image Sizes.

If you want to prevent certain image sizes from generating upon your uploaded images, you could use the filter intermediate_image_sizes to remove the generation of certain image sizes.

Example

function mbe_filter_image_sizes($sizes){
    unset($sizes['thumbnail']);
    unset($sizes['medium']);
    unset($sizes['large']);
    return $sizes;
}
add_filter('intermediate_image_sizes', 'mbe_filter_image_sizes');

If you wanted to add your own image sizes you could do so using the function add_image_size();

Example :: Fixed Width and Unlimited Height

add_image_size('f_width_u_height', 620, 9999);

Example :: Fixed Height and Unlimited Width

add_image_size('f_height_u_width, 9999, 620);

Example :: Fixed Width and Fixed Height

add_image_size('f_width_f_height, 620, 620, true);

Note: If you want the changes to reflect upon existing images, you will need to either re-upload the images or, use a plugin.