Compared to how big the Assimilation Cube was compared to Fed ships in BOBW and FC, the Tactical Cube IS smaller when you see how it compared size-wise to VOY. Having the name "Tactical" Cube still could mean its weaker. Assimilation Cubes are for the major assimilation of species, while Tactical Vessels are for the earlier military engagements but aren't fit for assimilation duties themselves.
It would've been better for the Borg to be more like the Zerg Swarm: Every single Assimilation Cube would be a Collective unto itself, acting on its own for the greater Borg goal of assimilation but with it's own interpretation of how to achieve this. There wouldn't be one single "Big Voice" controlling all Borg vessels, but a different "Big Voice" for each one. Also, the Collectives could be at odds with one another; attacking and assimilating other Borg into their Collectives to increase their own power and numbers.
This would handily explain why that One Cube operated on its own in BOBW: It was the decision of that one Cube and that one Cube alone, and it didn't bother telling other Collectives what it was doing. So until VOY showed up in the DQ the rest of the Borg wouldn't have been aware of the Feds, but they'd be too busy battling one another for a real joint-invasion (if they even cared enough about the Feds).
It would've been better for the Borg to be more like the Zerg Swarm: Every single Assimilation Cube would be a Collective unto itself, acting on its own for the greater Borg goal of assimilation but with it's own interpretation of how to achieve this. There wouldn't be one single "Big Voice" controlling all Borg vessels, but a different "Big Voice" for each one. Also, the Collectives could be at odds with one another; attacking and assimilating other Borg into their Collectives to increase their own power and numbers.
This would handily explain why that One Cube operated on its own in BOBW: It was the decision of that one Cube and that one Cube alone, and it didn't bother telling other Collectives what it was doing. So until VOY showed up in the DQ the rest of the Borg wouldn't have been aware of the Feds, but they'd be too busy battling one another for a real joint-invasion (if they even cared enough about the Feds).