Some of the extra space may be down to unreused (but reusable) space allocated and freed during the doubling of the STASH hash as it grows.

Devel::Peek shows that stash entries created by coderef assignment to a GLOB are missing the MAGIC part of a GV:

use Devel::Peek;
sub foo {
    print "in foo\n";
}
*bar = \&foo;
print "----\nfoo:\n----\n";
Dump(*foo);
print "----\nbar\n----\n";
Dump(*bar);
__END__
----
foo:
----
SV = PVGV(0x1765790) at 0x1731550
  REFCNT = 3
  FLAGS = (RMG,MULTI,IN_PAD)
  MAGIC = 0x1732e80
    MG_VIRTUAL = &PL_vtbl_backref
    MG_TYPE = PERL_MAGIC_backref(<)
    MG_OBJ = 0x1725238
  NAME = "foo"
  NAMELEN = 3
  GvSTASH = 0x1705800   "main"
  GP = 0x1732870
    SV = 0x0
    REFCNT = 1
    IO = 0x0
    FORM = 0x0  
    AV = 0x0
    HV = 0x0
    CV = 0x1725238
    CVGEN = 0x0
    LINE = 4
    FILE = "f.pl"
    FLAGS = 0xa
    EGV = 0x1731550     "foo"
----
bar
----
SV = PVGV(0x17657c0) at 0x1725208
  REFCNT = 3
  FLAGS = (MULTI,ASSUMECV,IN_PAD)
  NAME = "bar"
  NAMELEN = 3
  GvSTASH = 0x1705800   "main"
  GP = 0x1759a40
    SV = 0x0
    REFCNT = 1
    IO = 0x0
    FORM = 0x0  
    AV = 0x0
    HV = 0x0
    CV = 0x1725238
    CVGEN = 0x0
    LINE = 5
    FILE = "f.pl"
    FLAGS = 0xe
    EGV = 0x1725208     "bar"
[download]

I don't think the sizes reported by Devel::Size are accurate since the stash entry created by coderef assignment is reported as being bigger.
Go figure.

use Devel::Size qw(size total_size);
sub foo {
    print "in foo\n";
}
print "size foo: ",size(*foo),"\n";
print "total_size foo: ",total_size(*foo),"\n";
print "size bar: ",size(*bar),"\n";
print "total_size bar: ",total_size(*bar),"\n";
print "difference: ",size(*foo) - size(*bar),"\n";
__END__
size foo: 7034
total_size foo: 7034
size bar: 7186
total_size bar: 7186
difference: -152
[download]