Initial index of views is highly fragmented

function(doc) {
  if (doc.a === 'b') { emit(doc._id, null); }
}

This view, running on CouchDB 1.2.1, takes 16 minutes to index from scratch on an inactive database, creating a 2.5GB file.
After compaction, the file is then only 157MB.

Can we do something about this waste of disk I/O, such that the initial index is smaller to start with?