1. Lower gravity allows volcanoes to grow higher if you have them. 2. Mars is large enough to have had volcanoes in its past (though it does not have active volcanoes now). 3. Weathering is very low on Mars, so large volcanoes stay large. 4. Mars probablly didnt have any (or much) plate tectonics in its past. This means that heat from the interior had only a few places to escape from (hot spots) rather than all sorts of places to leak out (like the Ring of Fire).