Why can Mars build volcanoes so much bigger than Earth's?

Because nothing moves. On Earth the crust slides over the hot spots beneath it, so a plume builds a chain of volcanoes rather than one mountain - the Hawaiian islands are a single hot spot and a moving plate, with each island the record of a period the plate spent over it. Mars has no plate tectonics, so eruption after eruption piles onto the same spot for hundreds of millions of years.

Lower gravity helps at the margin. A mountain can grow taller before its own weight starts to spread it outward, and Mars has about 38 per cent of Earth's surface gravity - but the tectonics is the main answer, not the gravity.

The result is Olympus Mons: roughly 600 kilometres across, some 22 kilometres high, with a footprint about the size of France. It is so wide and so gently sloped that standing on it you would not perceive a mountain at all, because the summit would be below the horizon. Crater counting suggests lava flows only a few million years old, which means it is probably dormant rather than extinct - the largest volcano in the Solar System may not have finished.