Those “gaps” are the margins. I am using a 27 inch monitor, not a laptop size or “17 inch” most computer people have. Thus, I get a very big picture.
Just like a “responsive” Theme (which yours is), if you make it small for an Ipad it adjusts the “fit” of the content, and conversely going “up” in screen size does the opposite. That is why you often find the images set to “100%” instead of a set width, like 1140px.
The “slider wide” has a bigger container than the regular “slider” area. You can place a wider image there and see how it looks.
On my website, I prefer the “margins” to basically frame the image.