The largest volcano on Mars rises 27 kilometers above the surrounding plain and covers an area roughly the size of Romania
Even if the Earth' s gravity were as low as the gravity of Mars is, no volcano of such size could exist on Earth, for the Earth' s crust, although of essentially the same composition as that of Mars, is too thin to support even a small fraction of that mass and would buckle under it, causing the mountain to sink
On average, volcanoes on Mars are higher than those on Earth.