A bit late, but here's the answer. We just need to add one more project stage to the pipeline at the end-stage.

{
    aggregate([{
        "$project": {
            "LSDNumber": "$LSD.LSDNumber",
            "year": {
                "$year": "$createdon"
            },
            "PointOfDiversionVolumeDetails": 1
        }
    }, {
        "$match": {
            "year": 2014
        }
    }, {
        "$group": {
            "_id": "$LSDNumber",
            "doc": {
                "$push": "$PointOfDiversionVolumeDetails"
            }
        }
    }, {
        "$addFields": {
            "GroupByDocCount": {
                "$size": "$doc"
            },
            "Cumulative": {
                "$reduce": {
                    "input": "$doc",
                    "initialValue": [],
                    "in": {
                        "$concatArrays": ["$$value", "$$this"]
                    }
                }
            }
        }
    }, {
        "$project": {
            "CumulativeVol": {
                "$sum": "$Cumulative.Volume"
            },
            "LSDNumber": 1,
            "GroupByDocCount": 1
        }
    }, {
        "$sort": {
            "GroupByDocCount": -1
        }
    }])
}