In any case, here's GX scaling the map if the Great Plateau truly is 1-2% of the entire world map

As we can see, it isn't entirely unreasonable to see the Great Plateau being 1-2% of the entire overworld map. Some of the pixels in the map not be totally explorable though since at 1.1%, the entire square map isn't fully covered yet.
And personally, the Great Plateau looks a little bigger, if not, the same size as New LA in Xenoblade X and that was only about a few percent of X's overworld as well. So BotW map size isn't unprecedented.
Edit: Found Xeno X overworld map:
Blue Circle in the southern pink region is NLA







