Much of Poor Richard's Almanack consisted of advisory sayings offered by Poor Richard (Benjamin Franklin) for the benefit of his readers.
In the case of the proverb you quote, Franklin is commenting on people who consider themselves the most important persons in the world. When someone is "wrapped up in themselves," that individual is not being aware of the feelings, opinions, contributions, or worth of anyone else. Because of this exclusion of others, such people are missing out on the enrichment of life that comes from exchanging ideas with others, adding input from outside sources when making plans or decisions, and in other ways expanding their lives and existences.
Instead of growing and becoming larger and more aware of the world outside of their own little bubble, self-centered people have only themselves for company - which makes for a very "small package" in terms of friends and companions along the way of life.