Warp geometry and power consumption. Shield battery retention. Phaser coil reserves. Etc etc.
Bigger ships need more power to do just about anything.
Monumentally bigger ships then need monumentally more amounts of power to "do just about anything" as potently as smaller ships.
A tactical ship is a ship that zips form one mission to the next doing odd jobs quickly as need be, and then move onto the next mission. Like we suspect was the idea behind Voyagers design.
The federation at the beginning of TNG can Build warships 20 times the size of the Enterprise but it chose not to, because it wasn't a good idea. They would have been too slow or gas hogs or most likely both, and then you have to wonder how well the shields would work in a firefight trying to cover 20 times the surface area? And really if they had enough surface area for another 40 phaser arrays, they'd be fools not to use it, and if they did have forty more phaser arrays than a galaxy class ship, imagine the resources and staff that that would take to maintain?
The job of an assimilation cube is to process thousands, tens of thousands of cities into raw resources and induct billions of individuals into the collective and then find assignments for all those new drones throughout the collective.
Not to actually strip a planet and move on, but to turn a planet Borg, building new factories and server farms and transwarp radio antenna relays to extend the collective influence...
Tactical cubes are smaller than Assimilation Cubes like the Galaxy Class Starships are smaller than Federation colony ships because they are a more efficient design. Faster engines, stronger shields and more powerful weapons etc for a relative expense of energy.
That's logical right?