QUESTION:
Why is the rover that will travel on Mars so small?

ANSWER from Cheick on November 20, 1996:
In our line of business, small is good. The weight of a spacecraft impacts
so much on the cost of a mission that we are happy whenever technology
enables us to reduce the size and weight of our instruments/spacecraft. 
The reduction in size does not necessarily mean reduction in the craft's
usefulness.

The Mars rover is small because it has to fit in a given space in the
lander and the whole spacecraft has to be a certain size to fit in the
launch vehicle and a certain mass to be launched by that rocket.

ANSWER from Donna Shirley on April 22, 1997:
Thanks for your question its a good one.
Sojourner, the rover, is so small because she's all we can afford and
because a bigger rover wouldn't fit into the Pathfinder lander. The rover
costs $25 million, which sounds like a lot of money but for a planetary
spacecraft is incredibly cheap.  For instance, $25 Million would pay for a
few commercials for the super bowl.

We have to make things small so we can use small launch vehicles (rockets)
to get our spacecraft off the earth and on their way to Mars.  The launch
vehicle for Mars Observer, the last mission we tried to send to Mars, cost
$300 million.  Pathfinder's launch vehicle only costs $60 million.  That's
because Pathfinder is much smaller and lighter than Mars Observer, so it
can use a smaller, cheaper rocket.

Donna Shirley
Manager, Mars Exploration Program