In mappings.json we exclude the context fields from the stored source document

"_source": {
    "excludes": [
        "context.*"
    ]
},

As the context fields are populated by the importer and updater, this implies that we can't reindex in situ (aka any change in the mappings or similar requires a reimport). I can see the space savings in doing this, but on the other hand it means that we can't evolve the mappings and related aspects of Photon in a "user"-friendly fashion.