We don't know that the Tactical Cube was a heavily armed version of the Assimilation Cube, we just know that it's a heavily armed Borg ship. Nothing in the dialog compares it to an Assimilation Cube and the visuals of it in comparison to VOY have it as MUCH smaller than an Assimilation Cube.
A Tactical Cube is 1500 M in diameter, an Assimilation Cube is 3040 M in diameter, the Tactical Cube is 1/4 the total size of the one from "Q Who" and BOBW.
As for why the Borg only send one Cube at a time: The writers overpowered the Borg, realized it and hoped we'd never complain about it. They got lucky.