Ok I get it now, you want the image to be bigger in the embedded Facebook post. Unfortunately, I don't think either of us can control this. Facebook decides on the layout and I don't think we can change it. On mobile and when there isn't a lot of horizontal space, Facebook chooses to display a big picture with its caption below, but when there's enough screen real estate (e.g. on desktop) it switches to a smaller square picture on the left side with its caption on the right.
It may be possible to influence Facebook's layout selection and I'll look into it, but I don't think it will change anything.