Weight penalties seem to be defined in the .hdv file for each car.
There is an [aidpenalties] section. I imagine the figures are precentages. Some aids are just on or off and others are off, low, medium and high so they have four values.
edit: Here are the values for the BMW F1 car:
[AIDPENALTIES]
TC=(0.00,0.00,0.00) // Weight penalties for using different
ABS=(0.000,0.005,0.010) // levels of aids. First value is typically
Stability=(0.000,0.005,0.010) // with the aid off so it should be 0.0.
Autoshift=(0.00,0.00,0.00,0.00) // Penalties should only be applied to
Steering=(0.000,0.005,0.008,0.020) // aids that the vehicle would not be
Braking=(0.00,0.00,0.00) // allowed to run with. Penalties should
Invulnerable=(0.00,0.00) // typically only be used if the aid improves
Opposite=(0.00,0.00) // laptimes for a decent driver.
SpinRecovery=(0.00,0.00) // Values are fractions of the total vehicle
AutoPit=(0.00,0.00) // mass, and are modeled as extra weight in
AutoLift=(0.00,0.00) // the fuel tank. Do not use negative values.
AutoBlip=(0.00,0.00)