Bill Rankin, 2008, 2012
This chart overlays the elevation histograms of the three planetary bodies for which we have high-quality satellite data.
Two quick observations:
1, Despite the fact that the radius of Mars is about half that of Earth, and the Moon is again half that of Mars, all three show roughly similar range of variation in elevation. The major exeption is the huge Mount Olympus on Mars, a volcano which can grow higher due to Marsís weaker gravity and drastically reduced atmospheric erosion.
2, The histogram for Earth clearly shows the presence of continents and oceans as two distinct geological features. The Moon, on the other hand, shows no such differentiation, while Mars falls in between. Astronomers are still speculating about the reasons for this distribution on Mars, but it may be evidence of past plate-tectonic activity, which is what maintains the land/ocean contrast on Earth.