private void recalculateSize() {
    this.totalSize = filesToCompact.stream().map(HStoreFile::getReader)
      .mapToLong(r -> r != null ? r.length() : 0L).sum();
  }
Recalculate the size of the compaction based on current files.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/compactions/CompactionRequestImpl.java