In the Bor-I mean Zorg episodes, the spheres are the smallest and weakest class of Zorg ships.
Actually, this is not true. Although the Sphere in the film First Contact was rather quickly defeated. This was probably done for movie purposes.
Let's look at Star Trek Voyager, for example. The Borg Probe that chased Voyager was roughly on par with the Starship. A Borg Sphere is much more powerful than an Intrepid-class Starship and a Borg Probe. In fact, Voyager was only able to defeat a Borg Sphere because it was armed with anti-Borg Transphasic torpedoes from the future.
In canon Star Trek the Borg Scout is the weakest ship. Then the Borg Probe, which is roughly as large as a Starship. Then the Borg Sphere, which is large enough to hold a starship in its hull. And then followed by the Borg Cube and the Borg Tactical Cube. The Borg Diamond or Octahedron is seen as the Borg Queen's special ship and is rather powerful. Probably more powerful than a Sphere but weaker than a Cube.
In Star Trek video-games the classes differ slightly. It has the Borg Interceptor (which is actually the Borg Probe) as the weakest ship. Followed by the, Borg Pyramid, Borg Diamond, Borg Assimilator,
Borg Sphere, Borg Tactical Sphere, Borg Cube, Borg Tactical Cube.
Both in the games and in the series the Borg Sphere is actually one of the stronger Borg vessels. It is used as a stronger long-range scout ship, or as a tactical warship.
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As for the Zorg, the Worker Sphere is the weakest. But the Tactical Overseer Sphere is the second strongest ship, just behind the Zorg 'Cube' (Matrix). Kind of like how the Borg Sphere is second only to the Borg Cube.