The Sphere is sent out with cubes to assimilate worlds and or on scouting missons. On the course of these scouting Missons, the sphere will encounter many ships and assimilate them. As they assimilate them, the ships will eveolve into a Cube. As it grows larger, it becomes more efficent and powerful with new additons and size.

The Cube therefore is only a Sphere that has been built up from a number of assimilations.

Or the Cube continues to grow as they assimilate other ships. This makes the Borg the scrap heap ship builders of the Galxay.

Those massive Borg cities in the sky are from cubes that have become so large that they are no longer efficent and are assigned other tasks, namely to join with other ships to form the city.

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I did'nt do it.