warner/pycryptopp — comment on commit 0c79f21f3157aabfa8a3f376181441e24a34114f
File: README.ed25519 line 34
By warner:

That sounds like a question for DJB, the author of Ed25519. To maintain compatibility with other implementations, we have to stick to the same format, of course. But we could always expand a 128-bit seed into the same 256-bit seed that Ed25519 requires, if we needed to conserve space and felt safe doing it.