Revolution slider uses Grid to determinate picture width and height, and based on this grid he make it responsive.
On the picture provided there is grid of width 1170px and height 500px, and revolution slider make images responsive with minimum of that width and height. If the images are not that big it will add custom padding.
Images can be bigger than 1170px but can’t be smaller.
There are 2 options for you, you can reduce grid size to your image width or you can make larger image.
I hope this will help you.