User blog comment:Suparockr/List of Plotholes throughout the series/@comment-4950455-20120725013903/@comment-76.78.130.227-20141107213115

But as pointed out, it's entire possible that - A. Cerberus already had a sizable fleet. B. Cerberus did not construct all these ships, but rather modified ships they acquired elsewhere. D. Cerberus controls many aerospace companies such as Cord-Hislop and has the capability to produce ships in large quantities. E. All or any combination of the above.

And as for Cerberus being a small and insubstantial group, they never were. The Systems Alliance and Citadel Council downplayed the threat, and The Illusive Man was never showing his full hand anyway. This is a group that owns multiple large space stations, bases and facilities across the galaxy, has agents everywhere, and may control huge swaths of multiple economies. They are a shadow organization, the whole point of that is to stick to the shadows. It's possible that even by the end of Mass Effect 3 we still haven't seen the full reach of Cerberus's power.

As for the Catalyst only caring about our galaxy, it is entirely possible that it was simply programmed only to care about our galaxy. That isn't so great a stretch. Furthermore, as has been pointed out by others, the Reapers may have gone to other galaxies and we simply don't know it. They are definitely still an enigma, we know more now about them since ME3 concluded, but there is still a great deal that we have no clue about.