Size is determined by the number of avocados that fit in a standard carton, called a “lug,” that weighs approximately 25 pounds when full. For example, a size 40 avocado refers to the fact that 40 avocados fit into that standard size lug. Therefore, the larger the size number, the smaller the avocado. Conversely, the smaller the number, the larger the avocado.
Every 25lb lug yields roughly the same volume of pulp. So you can make the same amount of guacamole with a box of 70s as a box of 48s!