NASA’s JPL has a nice photo of two engineers standing with three generations of Mars rovers. There’s quite a difference in size when you start with Sojourner (1997) which looks like a large radio-controlled car. Next up in size were Spirit and Opportunity (2004), which are about the size of a small four wheeler. Finally there’s this year’s Curiosity, which is about the size of a Volkswagen Beetle.
This is a small version of the image. [Bigger versions are available at the JPL website](http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA15279).