Every grocery store are different so it will be a bit trickier to determine who got the cheapest products, but all these grocery store have the same or similar tricks to get you spending more money than you should have.
Avoid products that are for single use or products that are packaged for convenience. Products that are in Economy-sized isn't always cheap, the only way to be certain is divide the total amount by its unit to know the unit price.
If you like bagged salads, you'll be better off buying them separately. You'll have more lettuce and more dressing than purchasing a single packaged salad. Cut fruits is also more expensive than if you purchase the whole fruit in terms of unit price.
Products that have new features, tend to be more expensive than the regular stuff. Like for example, a toothbrush with new bristle. Why not buy an ordinary toothbrush that can also do the job, or a juice drink with new easy to drink bottle or no slip bottle.
Another way to save huge on your grocery bills is always use coupons and buy store sales.