Good information on that can be hard to come by. Catholic Charities don't seem to publish much audited financial disclosure info. They probably do claim low overhead but without audited statements, hard to be sure. Heck, would be dicey even if audited.
Samaritan's Purse gets better rating that Red Cross at Charity Watch (A- versus B+), but Red Cross has lower overhead (11 versus 23%). Red Cross spends much more on fund raising ($30 per $100 donated, versus $13 per $100).