That's interesting because my experience is the opposite. Costco can swallow up an irrational number of people and still be pretty smooth inside. The parking lot can be packed and when you walk in the door it turns out to be fine.

Walmart, on the other hand, is the absolute pinnacle of shitty shopper behavior. People being rude, pushy, completely oblivious standing in the middle of the aisle, etc. At least in my area, Costco attracts a different kind of clientele.