Per object overhead in functional languages is usually lower since polymorphism is handled at the function level rather than the data level. You don't have to store class pointers etc, just the raw data and probably a type tag.

I can't find a reference for erlang's representation but I believe it has only a little more overhead than ocaml which is described here - http://rwmj.wordpress.com/2009/08/04/ocaml-internals/