Critics of big banks have raised "a legitimate question" of whether the marriage of commercial and investment banking is too unwieldy, a former JPMorgan Chase (JPM) executive said Wednesday night.

Heidi Miller, who retired as head of JPMorgan's international businesses in February after an executive shake-up last year, defended the utility of large, diversified financial services companies — and criticized the industry for not doing a better job of defending itself. Yet she acknowledged that some companies run the risk of becoming too big.

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