Why does union consume more memory if the argument is a set?

Why does union consume more memory if the argument is a set?

References

Reference NO 1

http://www.chemie.fu-berlin.de/chemnet/use/info/gdb/gdb_9.html

Reference NO 2

http://www.swig.org/Doc1.3/SWIG.html

Reference NO 3

http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html

Reference NO 4

http://www.stroustrup.com/bs_faq2.html

Reference NO 5

http://www.forbes.com/sites/johntamny/2013/07/21/the-unions-didnt-bankrupt-detroit-but-great-american-cars-did/

Reference NO 6

http://python.net/crew/theller/ctypes/tutorial.html

Reference NO 7

http://www.thalesians.com/finance/index.php/Knowledge_Base/Databases/Kdb

Reference NO 8

http://golang.org/doc/faq