There is the sentence:
"The default setting is big endian (MSB first). "
in the QDataStream documentation (as of Qt 5.4).

AFAIK big endian means "ending with big(est) (i.e. MSB) number part", thus either
a) The default setting is big endian (MSB last).
(alternatively The default setting is big endian (LSB first).)
or
b) The default setting is little endian (MSB first).
should appear.

In present state developer can not decide based on doc whether big or little endian is really used in implementation.