Why is Olympus Mons so big?

The main difference between the volcanoes on Mars and Earth is their size; volcanoes in the Tharsis region of Mars are 10 to 100 times larger than those anywhere on Earth. The lava flows on the Martian surface are observed to be much longer, probably a result of higher eruption rates and lower surface gravity.

Another reason why the volcanoes on Mars are so massive is because the crust on Mars doesn't move the way it does on Earth. On Earth, the hot spots remain stationary but crustal plates are moving above them. The Hawaiian islands result from the northwesterly movement of the Pacific plate over a stationary hotspot producing lava. As the plate moves over the hotspot, new volcanoes are formed and the existing ones become extinct. This distributes the total volume of lava among many volcanoes rather than one large volcano. On Mars, the crust remains stationary and the lava piles up in one, very large volcano.

For more on Olympus Mons:

3-D image of Olympus Mons (you'll need 3-D glasses!)

Earth and Space Network

University of Michigan