The Melty Brain robot is a modified Thwack Bot that allows 100% of the bots mass to spin, but unlike a traditional Thwack bot that spins in place, the Melty Brain robot adjusts the wheel speed on each rotation allowing it to effectively drive around the arena.
1-wheeled robots: Although the design may seem counter-intuitive, there are advantages to using a single large motor to spin the robot. In addition to the increased power and durability of a single motor, tuning a single wheel to control the robots movement is generally easier.
2 wheeled robots: These melty brain robots have a more typical design with two wheels to balance the weight of the robot as it goes through it's rotation. Although these can be just as effective as 1-wheeled robots, the additional friction from the second wheel as well as weight-differences between the two wheels can make tuning the melty-brain more difficult. One advantage of a two-wheeled Meltybrain robot is that it can revert to (more usual) tank-drive steering, e.g. to escape from corners.
Mechanical Melty Brain: Although not a true Melty Brain, there have been examples where a small amount of weight is dedicated to a central 'directional' bot that maintains contact with the ground and uses a cam to adjust the wheel angle during the robots rotation. (Y-Pout)
Rich Olson over at Nothing Labs has an open source melty brain project and has successfully used his approach with a variety of weight-classes.