Polymer nanocapsules have gained an important place as drug delivery vehicles for a myriad of biomedical applications. However, the influence of nanocapsule size, wall thickness, and porosity toward controlling the drug delivery efficiency of nanocapsular systems is not well understood. We report a facile template-mediated approach for the development of near monodispersed chitosan nanocapsules of different sizes, wall thicknesses, and porosities in a controllable manner. The ability of this approach to finely tune the structural characteristics of chitosan nanocapsules enabled us to systematically investigate the influence of capsule size, wall thickness, and porosity on their efficiency as drug delivery vehicles against mouse mastocytoma cells after loading them with curcumin, a natural lipophilic anticancer drug. This study establishes an important finding in the field of nanocapsule-based drug delivery systems that although several structural characteristics of a nanocapsule might be responsible in influencing their efficiency as a chemotherapeutic carrier, the size of the nanocapsules is likely to play the most important role in dictating the chemotherapeutic efficiency of such systems.