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 ...
Yes, I acknowledged that above.
... and the visuals of it in comparison to VOY have it as MUCH smaller than an Assimilation Cube.
Impossible, since there was no actual on-screen size comparison. There was as much direct comparison in visuals as there was in dialog: None.
As I just mentioned, there is no such thing as an "Assimilation Cube" in canon. That comparison chart cites Ex Astris Scientia, which in turn cites Doug Drexler, which Luminus already cited in this thread, and states that the tactical cube was meant to be an upgrade of the original cube.
Here is the Borg ship page on Ex Astris Scientia:
The class-4 tactical cube is about the size of a cube (variant 2), but more heavily armed.
And Variant 2 is described as similar size to Variant 1, which is the original from Q Who
So there you have it. The class-4 tactical is the same size as the original, and stronger. Can we put this to rest now?