I visited Darlington Farms new outlet store (they have a lot of leftover fundraising items) where many of their items are $1.68, except large candles, and I heard one of the women there tell her husband when he was griping about the collective cost of the stuff in their cart was “It is the large carts…they make you buy things…”.
When we were at Costco recently Marv had postulated the same hypothesis, where when you have a big cart you have the subliminal need to fill it.
KMOM14- Project 365: A-Picture-A-Day 
I also think that “when the cart is full you stop buying” therefor a larger cart will keep you buying, just my postulation