What people mean is that the Imperial Province is really big, which for this case is a very valid size unit. The exact mathematical size is not that important.
People should either have a proper sense of scale, or admit that they're exaggerating. To someone as anol as me, when someone boldly claims that the Rumare Island is 500 times the size of ingame Cyrodiil, I immediately think they don't know just how big that really is.