ShardStatistics::currSizeBytes is populated either with the total size of the shard, or with the collection size, depending on a namespace being passed or not. This was for historical reasons, and the only remaining usage of currSizeBytes is in MoveAndMergeChunksPhase, which requires the collection size.

We should either create a separate ShardCollectionStatistics, or simply fetch the collection sizes at the site where it is required.