Complex sentence: The larger of the three would have been 2/3rd scale of the Great Pyramid of Memphis, Egypt; the flanking structures would have been scaled at 2/3s the size of the main pyramid.
Split into simpler sentences: The larger of the three would have been 2/3rd scale of the Great Pyramid of Memphis, Egypt. Because the flanking structures would have been scaled at 2/3s the size of the main pyramid.