The result is an integer which is the number of bytes of heap storage currently used to represent the object. If the object takes up no heap storage (fixnum or character), then 0 is returned. Such objects are represented by an immediate value held in a single machine "word".
Certain Common Lisp objects are not represented by a single heap object; for instance, using
on a hash-table is misleading as the function returns the size of the hash-table descriptor, rather than the total of the descriptor and the hash-table-array. General vectors and arrays also have this property. All symbols are of the same size, since the print name is not part of a symbol object.
LispWorks User Guide and Reference Manual - 21 Dec 2011