SAN FRANCISCO — A Facebook privacy class action that a judge once said might be too big to settle is being attacked on appeal for providing “only” $20 million in relief.

Massachusetts attorney John Pentz is asking the U.S. Court of Appeals for the Ninth Circuit to throw out the deal, which provides $15 to each user who filed a claim over Facebook Inc.’s “sponsored stories” feature. That pales against the $750 in statutory damages for each violation, Pentz says, and U.S. District Judge Richard Seeborg compounded the problem by steering several million dollars more in a cy pres award to 14 nonprofit organizations.