The size classification of pecans is a USDA standard. Pecans come in all kinds of shapes and sizes so to take out some of the confusion, they came up with a set of size classifications. This classification is for in-shell pecans. There is another set of classifications for shelled pecans. Each variety differs in percentage of nutmeat by in-shell weight. A large nut that is 40% kernel will be a smaller nut than a large nut that is 55% kernel when shelled.
Of course we all want the most nut per pound but the larger the nut the more challenging it is to completely fill that nut under less than ideal conditions. In a dry year or a short season for us up north the more likely we are to have incompletly filled nuts on oversized pecans.
Medium, large and even extra large nuts will yeild a more consistent crop for those who do not irrigate than those oversized nuts.
Like many things in life, bigger is not always better. If pecans are to be used for home use and they are not completely filled, they are still usable and will shell and eat just fine. But, nuts to be sold will need to be completely filled to be marketable.
Oil which is the main component of pecan flavor is usually higher in smaller nuts. No one can tell what the nut size was in chopped nuts and small pieces. One rarely grades the flavor of a pecan pie based on nut size in the shell.
When trying to decide which kind of pecan trees to plant, these guidelines can help you make an informed decision. Think about disease resistance and ease of growing as equally or more important than nut size.