Permanenty delete some Google Drive files from the trash. After I've watched the Videos in my YouTube folder I remove them but they still take much space in my Google Drive.

permanentlyDelete.gs

/**
 * Permanenty delete some Google Drive files from the trash.
 */
function run() {
  var trashed = DriveApp.getTrashedFiles();
  while (trashed.hasNext()) {
    parseTrashedFile(trashed.next());
  }
}

/**
 * Delete the file permanently if it is a YouTube video to save
 * some space.
 */
function parseTrashedFile(file) {
  // remove youtube files
  if (file.getName().substr(-4) == '.mp4') {
    if (getFullPath(file) == '/YouTube/') {
      Drive.Files.remove(file.getId());
      return true;
    }
  }
  return false;
}

/**
 * Get the full path of a file beginning and ending with a slash (/a/b/).
 */
function getFullPath(file) {
  var path = '/',
      parents = file.getParents();
  while (parents.hasNext()) {
    path = '/' + parents.next().getName() + path;
  }
  return path;
}