Saul Perlmutter is an expert on Supernovae, type Ia in particular, and using these to determine the geometry of the universe, for which he shared the Nobel prize. He is also an expert on high redshift galaxy clusters. He has not, to my knowledge, and, more pertinently, to the knowledge of the NASA Astrophysics Data System, published any refereed papers on black hole mergers as detected by LIGO. It isn't his subject.   

You are probably referring to a paper by B.P Abbott et al., which states that in a merger of two black holes of 36 and 29 solar masses, three solar masses were radiated away as gravitational radiation. This apparently is consistent with standard general relativistic models, but this gets to be seriously difficult stuff and beyond my mathematical capabilities.

The end product of all this is a rotating (Kerr) black hole, and when you have rotation I think the area and volume of the event horizon depends upon the rotation as well as the mass, though what Keith says is true for a non-rotating (Schwarschild) black hole. The maths are very hard though. Make it magnetic or charged and then it gets even worse.