Укажи byte order:
In [2]: s.pack("IBBIHH", 1, 2, 3, 4, 5, 6)
Out[2]: b'\x01\x00\x00\x00\x02\x03\x00\x00\x04\x00\x00\x00\x05\x00\x06\x00'
In [3]: len(s.pack("IBBIHH", 1, 2, 3, 4, 5, 6))
Out[3]: 16
In [8]: s.pack(">IBBIHH", 1, 2, 3, 4, 5, 6)
Out[8]: b'\x00\x00\x00\x01\x02\x03\x00\x00\x00\x04\x00\x05\x00\x06'
In [9]: len(s.pack(">IBBIHH", 1, 2, 3, 4, 5, 6))
Out[9]: 14
Иначе читай про выравнивание:
Note By default, the result of packing a given C struct includes pad bytes in order to maintain proper alignment for the C types involved; similarly, alignment is taken into account when unpacking. This behavior is chosen so that the bytes of a packed struct correspond exactly to the layout in memory of the corresponding C struct. To handle platform-independent data formats or omit implicit pad bytes, use standard size and alignment instead of native size and alignment: see Byte Order, Size, and Alignment for details.
https://docs.python.org/3/library/struct.html#byte... 
