How is that possible? Ground Zeroes isn't even all that big of an area.
MGS2 had some vertical traversing too with multiple floors, so the map is probably a bad comparison there.
That said I think people are underestimating how big GZ was. It definitely felt larger than MGS2 to me. Maybe people feel it was smaller because it was all one area, there were no loading screens and it was more open rather than traversing thin hallways?