Black holes' radius is linear in proportional to their mass, they are not constant density, larger ones are less dense. Therefore, when a 5 solar mass BH merges

Black holes' radius is linear in proportional to their mass, they are not constant density, larger ones are less dense. Therefore, when a 5 solar mass BH merges with another 5 solar mass, the radius doubles, surface area quadruples, and volume goes up 8x. If they "never actually merge", SMBHs would not exist.