One of the world's largest shopping cart manufacturers has found that "if the cart is double the size of our regular one, we buy an astounding 40 percent more than we usually do."
Similar results are found with popcorn buckets at the movie theater: people finish the whole bucket, regardless of how huge the bucket might be. And when asked, they claim that the size of the bucket has no influence on how much they eat. Of course not! We eat when we're hungry and stop when we're full, right?
So many things influence shoppers subconsciously. And they're incredibly hard to study in a survey: no one would say that the cart size influences how much they buy. Of course not! We shop when we need things and stop when we have them.
This is why controlled experiments and direct observations are so important.