Because of the large quantities my family with 4 children is able to go to Costco once a month and purchase almost everything our family will need for the entire month this means we only need to go to the store one or two additional times during the month for things like milk and bread.

Saying that everyone eating there is indulging in overconsumption is a ridiculous overgeneralization. Not to mention people that are planning parties, bbqs, get togethers etc. Just because you can't think of any reason for people to need large portion sizes besides overconsumption does not mean others are so limited in their imagination.

We have a larger family and Costco combined with access to a decent grocery store that's within walking distance is great: get deals on larger quantity staples and milk, eggs and bread several times a week.