Where the Big Families Are

This map is somewhat deceptive.  The average American household size in 2010 was 2.5 people per household.  The smallest average household size is among the densely urban District of Columbia (2.11 people per household) and rural, sparsely populated North Dakota (2.3 people per household).  Utah with its higher percentage of children has the largest households at 3.1 people per household.  So, there is not a huge difference between 2.11 and 3.1.

This map though shows the statistical standard deviations between the states (with DC excluded because it is such an outlier).  Still, you are going to find larger households on average in California, Utah, Texas, and Hawaii.  The western states in general plus Georgia and the large metropolitan suburbs in Maryland and New Jersey also attract larger households.