Do you think that's possible via flat buffers or grpc?

Never heard of those :)

Do you have more details on how this can be done in python? Do you mean compresing the string or just converting the CPID from a string to binary?

Sure. Change User.cpidfrom string to bytes and assign using hex conversion:

>>> cpid = '5a094d7d93f6d6370e78a2ac8c008407'
>>> len(cpid)
32
>>> cpid.decode('hex')
'Z\tM}\x93\xf6\xd67\x0ex\xa2\xac\x8c\x00\x84\x07'
>>> len(cpid.decode('hex'))
16

It does make it more tedious to use but there should be a significant reduction in size.