code: func (d *Driver) DiffSize(id, parent string) (size int64, err error) {
  	rPId, err := d.resolveID(parent)
  	if err != nil {
  		return
  	}
  
  	changes, err := d.Changes(id, rPId)
  	if err != nil {
  		return
  	}
  
  	layerFs, err := d.Get(id, "")
  	if err != nil {
  		return
  	}
  	defer d.Put(id)
  
  	return archive.ChangesSize(layerFs.Path(), changes), nil
  }
docs: // DiffSize calculates the changes between the specified layer
  // and its parent and returns the size in bytes of the changes
  // relative to its base filesystem directory.