An osmotic delivery system having a space-efficient piston is provided. The enclosure has an interior holding the piston, a protein or peptide, and an osmotic agent. The piston is movable with respect to an interior surface of the capsule, and defines a movable seal with the interior surface of the capsule. The movable seal separates the osmopolymer from the protein or peptide. The piston has a recess that receives at least a portion of the osmotic agent. The osmotic agent imbibes liquid from a surrounding environment to cause the piston to move and, in turn, cause delivery of the beneficial agent from the capsule.