Peak memory usage appears to be around 4 times the string length on Python 3.3, and around 3 times the string length on Python 3.4.

For 3.4, the peak occurs while formatting the exception message;  presumably at that point we've got all three of (a) the original string, (b) the exception message string being built, and (c) some sort of temporary string used during formatting.

For the purposes of this issue, I'll update the constants in the bigmemtest decorators.