"Because of their size, parents may be difficult to discipline properly"

This quote is funny and slightly sarcastic. P.J. O'Rourke is saying that moms and dads can be challenging to discipline correctly since they are physically bigger than their kids. This suggests that parents may be more difficult to control or punish compared to their smaller kids. The quote is most likely meant to satirize the concept of disciplining parents, as it is generally the reverse in many homes.