However large or small their initial seeds, once black holes formed, somehow they evolved to become the massive black holes we see today.
Black holes grow in mass as they accrete in-falling surrounding matter, and while this is happening, we are able to observe them as quasars. X-ray emission is characteristic of active black holes, and a clear signature of their growth. The Chandra Deep Field image (right) is the most sensitive X-ray exposure ever made. Most of the sources are quasars powered by supermassive black holes located in the centers of galaxies. We observe very powerful quasars up to distances corresponding to a time when the Universe was only one-tenth of its current age, and we infer that they are powered by billion-solar-mass black holes. 'Mini-quasars' powered by some of the seed black holes, which formed at even earlier times, but they are currently out of the reach of our telescopes. Not all black holes in the Universe are quasar though, especially at later cosmic time, like today. 'Black', quiescent black holes exist in the centers of many galaxies, including the Milky Way. These are black holes in galaxies that we can study very well, contrary to the case of quasars, where the light from the quasar itself "outshines" the host galaxy.
In general, we can think of today's massive black holes as weighing about 1/1000 of the host galaxy bulge, the roughly spherical component that dominates the central region of galaxies. Scaling relations have been identified between massive black holes and many other properties of the host galaxies, pointing to a joint galaxy and black hole cosmic evolution. When and how is the correlation established? We can envisage three possibilities. Massive black holes BHs may have grown in symbiosis with their hosts. Or, the black hole may have dominated the process, with the galaxy catching up later. Or, the galaxy grew first, and the black hole adjusted to its host. The constraints on the relation between black hole masses and the properties of their hosts at earlier cosmic times provide conflicting results. Such studies are fraught with difficultiees, but, still they are crucial to understanding how black holes evolved over cosmic time.
Understanding the role of mergers, gaseous flows from the cosmic web, secular processes and the back reaction caused by the radiation emitted from quasars in both growing the galaxies and growing the black holes is one of the most interesting topics of astrophysical research today.