>>11573225The radius of a black hole's event horizon is directly proportional to it's mass.
Say you merge objects the size of earth. There might me some additional compression due to gravity, but the resulting object would have a radius ~1.26 times that of the two initial objects.
If you merge two black holes of equal mass, the new black hole has a radius twice that of either initial one. Their horizons expand just by being near eachother, because of their proximity's effect on the curvature of space.
When you add normal mass to a black hole, you can think about it like that. The mass falling in is accompanied by a curvature of space that has an effect on the size of the event horizon. The black hole grows to the extent that the image of the infalling object is frozen and faded. Keep in mind, the redshift is extreme.
An aside to keep in mind, that might help with visualization, is that the the redshift is extreme. The smaller the black hole, the faster the light from the infalling object will be stretched out of the visible range, and out of the detectable range of any antennas that we have very quickly. It will appear black, for all intents and purposes.
