If a trim fitting diaper is important to you then the "perfect sized" diapers may be suited best for you—most babies only use 2 sizes of diapers (small and medium). If a diaper that will last from birth to potty, and will stand up to three years of use and possibly more is what you value, and you are willing to sacrifice some trimness for this—then the "One Size" may be your diaper of choice.