Yeah seriously, people think it's big because it is foggy, but it's not at all. It's quite a small area. Saying Soul Cairne will be that size is laughable.
But for a cave, it is rather sizable. Though i do hope not the size of the Soul Cairne.
Fun fact. From a technical standpoint, Blackreach is actually considered an "exterior" worldspace, and not an interior like most all the caves and ruins. The East Empire Trading Co cave is also technically an exterior.