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
The crust of Mars, at least at certain points on the planet, is thicker than the crust of the Earth.