ex-dyson robotic engineer here.
Correct - low weight means you have less force to apply to the wheels and hence achieve grip. The design of the aperture where the air goes into the vaccum (the soleplate) has to be optimised between flicking the carpet fibres in the right fashion to achieve good dirt pickup and not impeding sliding across the carpet too much.
The design of the tyres is critical. If there is any slip in the tyres you have a problem. Even though the attitude and trajectory of the robot is governed by the information coming from the camera it does also rely on feedback from the wheel speed sensors. If you have slip on one side and not the other then you will tend to spin around and not drive in a straight line. Adding more power to the slipping wheel merely exacerbates the problem.
You'll notice the treads on the tank tracks are chunky. This is to give maximum grip on thicker pile carpets, however this kind of carpet tend to be "squishy" and allow the wheels to float somewhat on the carpet surface no matter what design of tyre. So this is probably where they are having problems. Also you'll notice the wheels are not rigidly mounted to the robot chassis, they have suspension. But unlike car suspension the bottom of the robot does take some of the weight and it touches the floor. If you have more mass in the robot it gives you more latitude to distribute it between the wheels and the soleplate.
Its a careful balancing act and all of these variables have to be tuned based on the amount of suction your vacuum motor is providing.