Joe: Lisp heap objects are not of fixed size. Pointers to objects contain tag bits, with some of the common types completely contained within in the tag. So cons cells (the basic 2-tuple linked list element) are literally 2 bare words in memory with zero overhead. The fact that it exists as a cell is purely held in the tag bits of pointers reaching that memory location, not on the object itself. More complicated objects obviously have some actualized overhead, generally one word. Of course, objects themselves have no inherent mutexes, hashes, etc, that languages like Java do. Those would be user-managed fields.
Just something to toss in with the “needs thought” pile. Some of these likely do not affect the core functionality, but might be orthogonal add-ons of varying intensity.