There is a W3 (not-yet-released) standard named EXI (Efficient XML Interchange).
Should become THE data format for compressing XML data in the future (claimed to be the last necessary binary format). Being optimized for XML, it compresses XML more ways more efficient than any conventional compression algorithm.
With EXI, you can operate on compressed XML data on the fly (without the need to uncompress or re-compress it).
EXI = (XML + XMLSchema) as binary.
And here you go with the opensource implementation (don’t know if it’s already stable):
Exificient