That's why I say that the Tactical Cube is weaker than the Assimilation Cube.
Except that nothing on screen corroborates that notion.
In fact, the Tactical Cube was seen as VERY large - that thing was a monster compared to Voyager.
Might be a bit smaller than the 'Assimilation Cube', but doesn't mean it's any less powerful - only that their primary functions are different, and even then, if the Assimilation Cube is larger, then that can easily be interpreted in a way that it needs extra space for assimilated drones and possible storage of technology.
Also, Voyager's battle with the Tactical cube came after several key factors:
The Drone 'One' enhanced Voyagers defensive/offensive systems, 7 of 9 was present on board which probably helped in the battle either way, and SF sent Voyager tactical upgrades in the episode 'Lifeline'... plus the ship was already modified by the crew quite a lot by the time the battle with the Tactical Cube happened, so it's really no wonder the ship was able to hold out as long as it did - and their attacks were surgical in the first battle -they were likely protecting the Warp drive from being knocked out.