Probably the first point of comparison is the volume of contents of the packages you are comparing. Simply looking at the size of the packages may not be a meaningful comparison; you would want to determine the number of ounces of actual product in each package.
Another important point could be consideration of how well the different packages protect the contents. Particularly with crisps or chips, some packages may be designed to provide more protection to the product so you can buy intact crisps instead of crumbs. Another part of protecting the product would be finding out any expiration dates that may be on the different packages.
You may want to consider the ecological impact of the packages you are comparing. What materials and processes were used in the manufacturing of the packages? How easily will the packages be recycled?