It would have to refine a lot of Neutronium to "grow".
Spock traced its path as coming from another galaxy and so it had traversed inter galactic space. Unless the Borg orginated outside the Milky Way Galaxy, the weapon was intended to be the final say in some other war.
Propulsion was via "Total Conversion Drive" which may have allowed Warp Speed travel.
Defences were the hull descibed by Spock as being "Immesurably Thick and comprising of pure Neutronium", a Sub Space jamming field and an Anti-Matter Potency Neutaliser. Actually in the SFC senarios involving the Doomsday Machine, Photons and Warp Power \ Drive should be off line. Now that would make the battle more intersesting!!
Offensive weapons were the Anti-Photon Beam (lacking in the SFC Doomsday Machine so I added 8 x Ph4) and a powerful tractor beam.
Spock also pointed out that the Doomsday Machine was slightly faster than the Enterprise when at Impulse. "It is gaining on us!!"
Commadore Decker babbled on that the machine was "MIles Long with a mall that could swallow a whole fleet of starships!!". The SFC version does not do justice to how big the thing was shown next to the Enterprise when viewed from the view sceen in auxilary control onboard the USS Constellation.
Now if someone could manage the ghostly blue shimmer that was in the origninal TOS episode, there'd be something!!
If you've not seen the episode, ten you've missed out on one of the best episodes in the TOS series. You even see Kirk about to loose spicter muscle integrety in the final moments of the Constellation's existance!!
It's a shame that they couldn't have the Doomsday Machine music in the SFC senarios involving it. Next to the original Star Trek theme, it is the next most familiar music in the TOS sries.