Well, I think you will find most fluid dynamics researchers will agree that a "drop" is a body of fluid where surface tension is non-negligible. We might argue over whether that limit is 25 mm or 5 mm or whatever for a given fluid, but we all agree it's a lot less than 1 meter for water.
Well, in this case the concept is that a drop-like thing has relatively large surface area compared to volume. Translating e.g. to people, it could mean many/all of the people in a group have ties to outside the group. Maybe "drop" is a neighbourhood and "sea" is a country.
