Buying in bulk not always cheaper

So I was buying some OJ at the grocery store and had the option of the bigger or smaller bottle. I instinctively went to grab the bigger bottle thinking it'd be cheaper per oz, but I thought I should check the label. Turns out the smaller bottle was also cheaper per oz. Go figure. Of course, there was the milk that cost as much to buy a half gallon as a full gallon; so the half gallon was twice the price per oz.