Here is some info I found on BYO.COM
Pump up the volume
Astute readers may ask about the increase in volume from the extract addition. With a typical water content of 20%, liquid malt extract has a calculated volume of 10.6 fl. oz. per pound (692 mL/kg). At first thought, the additional volume contributed by dried malt extract would seem negligible. However, it is real and measurable, and for those who are sticklers for accuracy, we will mention it here. When sugar is dissolved in water, there is an increase in volume. Because of molecular interactions, the formula is not strictly linear, and the result increases slightly along with the specific gravity of the solution. A very reasonable average value for the gravities used in brewing is 9.42 fl. oz. per pound (614.2 mL/kg). That is, 1.0 lb. (0.45 kg) of dried malt extract added to the wort will increase the volume by 9.42 fl. oz. (or 1.0 kg will increase the volume by 614.2 mL).
Here is the link for the article:
http://byo.com/stories/techniques/artic ... ed-brewing