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Well, then I guess you need to specify your requirements
more clearly. XML-RPC was precisely developed to be
something simple for primitive types and structures that is
sufficiently  well-specified to allow interoperation between
various languages.

I don't see why extending the data 'by an order of
magnitude' would be a problem per se, nor do I see why
'requiring a complicated parser' is a problem if the
implementation already does all the unpacking for you under
the hoods.

Furthermore, I believe it is simply not true that XML-RPC
expands the representation by an order of magnitude. For
example, the Python Integer object 1 takes 12 bytes in its
internal representation (plus the overhead that malloc
requires); the XML-RPC representation '<int>1</int>' also
uses 12 bytes.
In short, you need to say as precise as possible what it is
that you want, or you won't get it. Also, it may be that you
have conflicting requirements (e.g. 'compact, binary', and
'simple, easily processible in different languages'); then
you won't get it either. For a marshalling format that is
accessible from different languages, you better specify it
first, and implement it then.