It's not that impressive when you see how they do it. It's actually really easy to make very large perfect circles and patterns from the ground using pieces of string and precise measurements. Then you stamp down the crops using a wooden board. And with a group of people it might only take a few hours to complete.
I remember thinking the Nazca Lines were incredible until I watched a show where a guy reproduced them exactly, from the ground, using just string and tools available to the people at the time.
Well I still think they are incredible but knowing how they made them but it took some of the mystery and amazement away.