The above photo, courtesy of UniverseToday.com, shows all three generations of NASA Martian Rovers. The tiniest is Sojourner (1997), followed by the twins Spirit and Opportunity (2004), and finally Curiosity is the largest on the left. As you can see, they've been getting larger with age! This is mostly to hold increasingly complex laboratory and support systems needed to properly analyze the Martian surface.
At 1:24am EST on Monday morning, Curiosity will enter the Martian atmosphere and attempt to land. All of our fingers are crossed!