A compartment system with flexible volumes of space for use within a bag. The compartment system includes a primary compartment and a secondary compartment defined by the inner lining of the bag. The primary compartment is disposed adjacent the secondary compartment such that the sum of the volume of the primary compartment and the secondary compartment remains constant. The total volume of the primary and secondary compartment is approximately the volume of the bag. Increasing the volume of the secondary compartment space simultaneously decreases the volume of the primary compartment space such that the bag maintains a consistent exterior appearance. Access to the secondary compartment is via a compartment access flap disposed on one of the sides of the bag. The secondary compartment can be any size, shape or orientation. More than one secondary compartment can be used.