It’s because it’s still in 700px height regardless of device view. Please check these images to explain it.
This is contain, which means the image should be contained in FULL VIEW, hence, the gap is unavoidable since there is a difference between image height and 700px.
And this is cover, which means to cover the entire container including spaces, hence clipping the image.
And this is 100% width and 100% height, stretching it up to 700px.
Is that what you’re trying to achieve? Stretching it?
It’s unavoidable since you’re fixing your section height to 700px while the width changes. And there is no fix to it unless you remove the 700px. Again, there will still spaces or clipping since the content still defines the height, and that height depends on the amount of the content.
Another option is not using the section background, instead, use the revolution slider with the responsive setting. That should respect the correct aspect ratio of the image per device view.