In voyager we see the Borg have thousands of ships of varying sizes and control a vast area of space. Voyager is able to take down spheres and small cubes.
Yet in Wolf 359 a single cube attacks and destroys hundreds of star fleet vessels. If a single cube is able to have that level of effect why didn't the borg commit a larger fleet?
You have the same issue in First Contact, they only commit 1 cube.
Considering how difficult the federation finds holding them back, attacking with 3-6 cubes would seemto assure victory