KiCad export board area only
It is likely that you have some design element that exists outside of your expected board area.

You can check this by running DRC and looking for unconnected items (if they are copper) or by enabling all of the other layers and visually inspecting the board area.

KiCad will choose the "Board Area Only" based on all of the elements in your design, not just the visible elements, so that will include layers that are not output in your SVG.