Wall Street is again abuzz with the notion of breaking up a big bank—but it may be focused on the wrong one.

Goldman Sachs last week analysed potential gains from breaking up JP Morgan. The impetus: a recent Federal Reserve proposal to increase big-bank capital buffers that would hit JP Morgan hardest. Yet the market isn't crying out for such a breakup. JP Morgan's shares trade above book value, an indication its current structure isn't viewed as destroying value. And only twice over the past 10 years have its shares traded below tangible book value.