Just to put some meat on these bones, I think it is big enough. A Borg cube is 3km on each size (stated on screen) so each deck would be 9 sq km. If an alcove and the piece of corridor in front of it is 1 sq metre, that's about 9 million alcoves per deck.
Vertically, I'd estimate each deck is about 5 metres tall from what we've seen, so 3km would be about 600 decks.
This gives us roughly 5 billion alcoves on a Borg cube.
Plenty of space for a colony sized population, even if half the internal space is actually taken up by machinery or resource stores. Most colonies don't have populations in the millions, let alone billions.
Intriguing. Still, presumably the cube would have to have enough empty alcoves to begin with.
Although the idea that the Borg normally assimilate people is a retcon from
Voyager. In TNG, the Borg were mainly interested in technology, and "Q Who" showed that drones were incubated from infancy. The drones in "I, Borg" and "Descent" had no pre-Borg identity that they regained upon liberation, but were complete blank slates, never having been anything but drones. Assimilation was something done in special cases like Locutus, and in
First Contact to replenish the depleted numbers of the Borg that escaped in the Queen's sphere when their cube exploded.
But
Voyager's writers either forgot that or, more generously, decided it was more interesting to stick with the idea of the Borg as infectious zombies. VGR even claimed that Borg increased their numbers
exclusively through assimilation and never procreated, which just doesn't make any sense -- that's like subsisting by hunting and gathering rather than agriculture, a far less productive and reliable method. And what's the point of assimilating a species's biological distinctiveness if you lose it again once all the drones assimilated from that species die of old age? Obviously they have to reproduce their drones.
In the TNG paradigm where the Borg mainly assimilate technology, let's see... If you've got 600 x 9 = 5400 square kilometers of deck space, that's about what you'd get if Manhattan were totally covered in 90-story buildings. That's room for a fairly hefty amount of tech, enough to absorb several modest colonial cities, though not nearly enough to totally de-urbanize a heavily populated planet.