The invention provides highly concentrated chitosan-nucleic acid polyplex compositions and dispersions, and methods for producing the compositions and dispersions. Methods of mixing the chitosan-nucleic acid polyplexes include an inline mixing of chitosan solution and nucleic acid solution, followed by further concentrating the dispersion of chitosan-nucleic acid polyplexes, optionally with an aggregation inhibitor. Further provided are methods for altering the diameter of chitosan-nucleic acid polyplexes.