various densities have been used, with the most common now being 300 and 400 cells per square inch. All other dimensions being equal, converters with 100- or 200-cell densities offer improved air flow capacity, but reduced emissions control efficiency. In many cases, it's necessary to significantly lengthen a 100 cell converter to achieve the necessary levels of pollutant control. That additional length reduces flow capacity, so it's possible to end up with a 100-cell converter that does not flow as well as a 300-cell model.
Taken from a tech sheet i studied a few years ago about cats.