Why does Python's float raise ValueError for some very long inputs?

On my Python 2.7.9 on x64 I see the following behavior:

>>> float("10"*(2**28))
inf
>>> float("10"*(2**29))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: 10101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010
>>> float("0"*(2**33))
0.0
>>> float("0." + "0"*(2**32))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Unless there's some deeper rationale I'm missing this violates least surprise. When I got the ValueError on "10"*(2**29) I figured it was just a limitation on very long strings, but then "0"*(2**33) worked. What's going on? Can anyone justify why this behavior isn't a POLA bug (if perhaps a relatively irrelevant one)?