As engineers we want to build things that have a low center of mass. They are more stable and there are less forces to deal with. In the case of the BiBli robots we started with a very tall (48 inches) Big BiBli robot that was built on a ROOMBA base. One of the original requirements was to have something that could greet a child when it came into a library or classroom. This required it to be tall which worked in the end but which has a couple of issues.
1. The further away from the plane of acceleration you place a mass, the more force it requires to start and stop the robot. In other words, if you have a big head with a bunch of hardware it gets harder and harder for the ground-level motors to get the robot moving.
2. This impacts battery usage, stability and speed.
The physics behind this are quite simple. You can actually create a moment diagram to define the acceleration:
Where M = Force x Distance
The shorter the distance, the less force it takes to get the robot accelerating!