The ratio for Citigroup's CEO pay to its employees' average pay is 17:1. Not that big compared to other large companies. It's comparable to Microsoft (13:1) and General Motors (20:1). Huge discrepancies can be found at Wal*Mart (717:1), but then Wal*Mart employs 2.1M workers. The biggest difference I've found is UnitedHealth Group (1737:1).
source:
http://www.payscale.com/ceo-income/fortune-50