Among the top 15 servicers of subprime mortgages, only one grew its portfolio over the 12 months prior to the end of September, according to a new ranking and analysis by Inside Nonconforming Markets. The increase was posted by Citadel Servicing Corp., which is boosting its portfolio with originations of nonprime mortgages. Citadel handled a subprime portfolio with an unpaid principal balance of $1.80 billion as of September-end. The portfolio grew ... [Includes one data chart]