db.collection.stats(scale) - old behavior. retain for compatibility.
db.collection.stats(options) - options format:

{
    scale: N, // scales sizes in result
    indexDetails: true/false, // toggles display of indexDetails in result. Defaults to false
    indexDetailsKey: {x: 1}, //  requires indexDetails to be true. filters indexDetails subdocument to display single index corresponding to given index key (uses getIndexes() output)
    indexDetailsName: "x_1", //  requires indexDetails to be true. filters indexDetails subdocument to display single index corresponding to given index name (uses getIndexes() output).
} 

Needs to be updated at db.collection.stats()

See linked.