Mongos aggregation execution error

  mongodb, question

I want to use mongo’s aggregation to calculate uv, but the following command, as shown in figure:
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Error reported, aggregationresultsexceeds maximumdocumentsize (16mb)

The document mentions this problem as shown in the figure.http://docs.mongodb.org/manual/core/aggregation-pipeline-limits/,
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Please help me see
Thank

There is too much data to process. You can use $project to select the next field and then group.

{$project:{
 xxx : 1,
 yyy : 1,
 }
 }  ...