Perhaps instead of manually redrawing the entire island at every resolution, it would be easier to just save the island without any outlines as a separate image, and all outlines, one for each resolution, as individual images. Then the island body can be scaled to desired size within unity and the appropriate border could be layered on top of it depending on the scale. Or alternatively, if a smooth or continuous zoom is needed, a 1 pixel wide border could be procedurally added around the island body always. I don't know how it can be done in unity, but I am assuming it is possible.