Part of this complexity comes from what IMO was a mistake on Java's design (which was AFAIK copied by C#): the base Object class does too much. It has equality comparison, string conversion, object hashing, and a per-object re-entrant lock. Other than equality comparison (which is also bad because it contributes to the perennial confusion between identity equality and value equality), these need extra storage for each and every object in the system (string conversion contains the object hash code as part of its default output). Some tricks are used to avoid most of the space overhead for the per-object lock, at the cost of extra complexity.

I always kind of found it interesting that specifying the size of object fitters was part of the JVM specification. This sort of seems like an arbitrary implementation detail, since the programmer will never have to know anything about it.