Parents are notoriously irrational shoppers; they are exceeded in nonsensical purchases only by grandparents. There isn’t much to be done about this; "irrational" hope and fear play far too great a role in the "decision" to have children in the first place. This trend is exacerbated, however, by the children’s clothing industry. Baby clothes are often purchased by people only distantly related to the intended recipient and who therefore have only a vague notion of the child’s size. As children grow so fast, it is tempting to discard considerations of economy when shopping for their clothes. Dressing small children so closely resembles dressing dolls, in fact, that considerations of practicality are usually trumped by visions of "cuteness."
As inevitable as these factors are, they should not be used as an excuse not to revise the system of clothes sizing for children. Currently, clothes marketed for children under three years of age are categorized by age. It doesn’t take a pediatrics internship to know that children of the same age vary wildly in size. Furthermore, different manufacturers disagree as to how large to make clothes for a given age. Some manufacturers are even inconsistent with themselves in this regard. Consumers fail to punish these deficiencies due to the irrationality discussed above, but I think there’s room for a manufacturer who employs a more rational sizing system. The obvious determining attribute is weight, the standard already employed in marketing diapers and (as dictated by law) car seats. I don’t imagine there’s any dissuading Aunt Ginny from buying that hot pink bunny suit, but were it labeled with a weight range there’s a better chance it would actually fit come Easter.
Update: A reader writes that many children’s clothes are labeled with a weight range, but no one pays attention to them. Alas.