In Python strings are objects, so that values is the size of the object itself. So this size will always be bigger than the string size itself.
From stringobject.h
:
typedef struct {
PyObject_VAR_HEAD
long ob_shash;
int ob_sstate;
char ob_sval[1];
/* Invariants:
* ob_sval contains space for 'ob_size+1' elements.
* ob_sval[ob_size] == 0.
* ob_shash is the hash of the string or -1 if not computed yet.
* ob_sstate != 0 iff the string object is in stringobject.c's
* 'interned' dictionary; in this case the two references
* from 'interned' to this object are *not counted* in ob_refcnt.
*/
} PyStringObject;
From here you can get some clues on how those bytes are used:
len(str)+1
bytes to store the string itself;- 8 bytes for the hash;
- (...)