When I shop I usually buy the product I like, but use the price tag to determine which "size" box is the best deal. Typically the larger quantity you buy, the better the "per ounce" or "per unit/each" price is, but sometimes its not... Cereal is an example. Sometimes the smaller boxes are on sale, and the price per ounce is cheaper than the big "bulk" box size, so you're getting more bang for your buck buying 2 small boxes instead of 1 large box.
That said, I usually just buy whatever brand I prefer anyway. We shop at Costco for the main stuff we can buy in bulk, like toilet paper, paper towels, dog food, chicken (party wings that come in 8 individually separated vacuum-sealed bags of 1lbs each, that we freeze), etc. Then we try to buy the non-bulk stuff like eggs, fruit, veggies, etc. either at a farmer's market or at King Soopers (Kroger).