It's kinda weird to compare ruby with ocaml for object overhead. The former is a dynamically typed language that fully supports all kinds of introspection/metaprogramming at runtime, while the latter is a statically typed language that has very limited introspection capabilities.
But the numbers are interesting anyway.
Objects in Ocaml exist outside the normal type system, and can be introspected at runtime (no type erasure).
Of course, nobody uses objects in Ocaml, so it doesn't matter!