This is by design: the choice of -6 is not arbitrary, but follows the standard that the Decimal class is based on. Quoting from http://speleotrove.com/decimal/daconvs.html#reftostr:

> If the exponent is less than or equal to zero and the adjusted exponent is greater than or equal to -6, [...]

"str" isn't parameterizable (at least, not without a major change to the way that Python works), so if you're using it you have to accept the compromises it makes. If you want finer control over the string representation of Decimal objects, use Python's formatting capabilities:

    >>> from decimal import Decimal
    >>> x = Decimal("1e-8")
    >>> format(x, 'f')
    '0.00000001'