Perhaps no one really cares, but just in case: the reason why such as monster could occur is that it happened in the very early stages of the Universe after the expansion had begun. The universe was much denser in mass then, affording great potential for gravitational collapse of matter into the supercompressed state of a "singularity". The word "singularity" indicates that according to Einstein et al, the mass would be compressed into a zero-volume state --- i.e. it would occupy NO space, just a point-of-force-focus. Paradoxically, the biggest things in the universe mass-wise, are also the smallest space-occupying-wise.
The second reason why we would expect such monsters way back then, is that the hyperdynamic state of early expansion would have been a chaotic situation, creating all manner of "unequal distributions" of mass in given space volumes --- "heavier" volumes creating earlier Black Holes, which would get a head start gobbling up more mass. {and forming cores of early galaxies}. As those galaxies continued to grow with their attracted matter swirling about, that matter would tend to drift to the center feeding the early monster.
Black holes, as I guess everyone knows, SEEM to have spatial size only because the force they project is so gravitationally "steep" that light cannot reflect or in any way traverse the nearby space, at a distance which is greater or lesser depending upon how much mass has been "swallowed" into the core point-of-force-focus. More mass swallowed means a wider radius of space surrounding it from which light [and all else] cannot return. But if true, what are the scientists talking about when they speak of measuring X-radiation, which is just a form of extremely shortwave light?
X-rays of extreme energy are thought to be a signature of the existence of a black hole which is still in a dynamic "feeding" process, but the X-rays do not come from the BH itself. As the gravitational force of the focus point attracts surrounding matter towards itself, that matter swirls about an axis and "down-the-drain" {one of the peculiarities of Creation is that every fundamentally physical object has "spin", even the BH}. During this swirl down the ultimate toilet, the matter [still on the outer edge of the disappearance point] interacts with its fellow doomed particles producing nuclear fusion quality heat and beyond. This action alters the matter to produce light of all kinds, thus the high energy X-rays --- some of which "escape" into space since they're not yet inside the "event horizon" [the "nothing escapes" radius boundary]. These X-rays sail across space at light velocity, finally telling us what has gone on, in this case almost 13 billion years ago.
Our scientists' estimates of EXACTLY what is going on, though, are theoretical. This is because if we're honest we cannot say that we fully understand BHs [or, weirdly, if they even exist as we model them]. We trust our current theory for their formation and nature, plus we guess that they are the only likely things which could produce X-rays of these high energies at such a constancy of power and location. Frankly, we don't know any of that for sure, even though it's a pretty good guess. The SIZE [i.e. mass content] of any BH is also a best-guess situation. Measuring mass at a vast distance is trouble, not only in getting the decimals right, but it depends upon the measurement of other things about the light source that one CAN see. For all that guesstimation, and why we make it, you'll have to sign up for a modern astrophysics class.
