shelve file space always increase!

[smalltalk]

>>> import shelve
>>> sf = shelve.open('e:/abc.db')
>>> for i in range(10000):
...     sf[str(i)]=i
...
>>> sf.close()
>>> sf = shelve.open('e:/abc.db')
>>> sf.clear()
>>> sf
{}
the abc.db is always 312k though i have use clear(), how can i shrink
the space?
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