The Tactical Cube is smaller than an Assimilation Cube, and the designation "Class 4 Tactical" implies it's also weaker.
Funny thing is, if VOY was always able to put together armadas and they always got wasted by the Borg in every encounter, and all of VOY's Borg stories were the same except they had an armada at their command, no one would complain. The quality wouldn't matter, the number of cannon fodder ships getting trashed would redeem those stories.
The Cube in "Q Who?" and BOBW was something meant for taking on entire civilizations and assimilating their important worlds, a "Tactical" Cube by comparison is going to be for tactical battles BEFORE the major assimilations happen. Softening them up for the Assimilation Cube's major attack.
The real problem was overpowering the Borg so much, making them so numerous along with the overpowering, and being lazy in not better defining them earlier on.
Doug Drexler, the man who designed the tactical cube describes the drawing that was chosen as basically "a Borg cube wearing a flak jacket!" Source
So, no, it is NOT smaller than a cube. Why would you even say that? Also, a Tactical Cube is described as a Class 4 tactical vessel that is a type of heavily
armed Borg cube utilized by the Borg Collective. Where did you get the idea that Class 4 somehow implies weak? The fact that it has hull armor already makes it more powerful than a cube and that's before we start talking about weapons capabilities and other features.
How do you know what the cube in "Q Who?" was for? Or what a Tactical Cube is for? There is no mention of purpose for these things on screen... ever. It is more likely that the Tactical Cube exists solely to combat Species 8472 or others like them, but this is just speculation.