The largest (by volume) structure at Calakmul, Campeche, Mexico:

From this spot, the pyramid actually looks much smaller than it really is. The part you see from the base here is actually a later addition (more accurately, collection of three additions) to the original pyramid, which then rises even further behind this bit in the front. In all, Structure II has a base covering 120 x 120 meters (394 x 394 feet), and stands 45 meters (148 feet) tall.