Anorexia can kill you. "Bigorexia" , unless you resort to drugs, will just leave you better looking, stronger and healthier. I don't see how the two equate.
Multifinality: they are at the core most likely the same psychopathologie, but result in different patterns of self-destructive behaviour.
Bigorexia, can have positive outcomes but at the cost of impeded functioning in (social) life, often bigorexics will go for drugs, lots of them! Think the Munzers of this world.
I agree that anorexia is often a more dangerous syndrome.