You concede that the tactical cube is the same size as the standard cube as seen in FC, yet still argue that it's weak
? The FC cube was a major threat that was beating the entire fleet to a pulp.
The FC Cube was weaker than the one in TNG: In TNG they Armada couldn't even scratch the Cube whereas in FC they were able to do damage to it before Picard told them where the weak point was.
The FC Cube was still smaller than the one from TNG.
So it all comes down to "Yes, I need them to spoon feed it to me that the Borg ships were weaker than the massive one from TNG."