Just as the mould size and shape have effects on slumping temperatures and strategies, so does the weight.
When slumping you are making use of the combined effects of gravity and the increasing softness of the glass. The same thing happens when you have a thick piece of glass as when you have a large span in the mould. As the weight of the glass increases, the temperature at which it will begin to slump is decreased. There is an inverse relationship between the weight and the slump temperature just as there is between increased span and slump temperature.
A 3mm piece will take more time or more heat to fully slump into a mould than a 9mm piece will into the same mould. Observation will give you the information on what the temperature differentials are.