The big utility merger announced today by Exelon Corp. and Constellation Energy is significant by just about every measure, but it's not as big as the corporate merger announced in January by Duke Energy and Progress Energy.

Chicago-based Exelon and Baltimore-based Constellation are proposing to create a combined utility that sells electricity and natural gas to 6.6 million customers in Illinois, Pennsylvania, and Maryland.

By comparison, Charlotte-based Duke and Raleigh-based Progress will have 7.1 million customers in six states when that merger is completed.