Cutting costs—for instance by closing branches—might take out 30% of Commerzbank's cost base, according to Magdalena Stoklosa of Morgan Stanley.
Should the pair come to talking about takeover terms,
Commerzbank's bargaining position looks far stronger than it could have dreamed of a few years ago.
Neither bank can be called highly valued: the stockmarket prices Deutsche at a paltry 24% of net book value and Commerzbank at 31%.
But so far has Deutsche fallen that its market capitalisation is now just twice Commerzbank's, against six times as much in 2013.
Even taking into account Deutsche's talent for attracting trouble, for Commerzbank the ratio may not get better than this.
Besides extra bulk and the chance to serve more of the Mittelstand—Germany's myriad, mainly family-owned, companies—
Commerzbank offers Deutsche improved funding. Commerzbank relies more than Deutsche on deposits,
which are cheaper and stickier than funds from financial markets.
Its 311bn euros ($363bn) of deposits at the end of September were worth 63% of its adjusted assets; Deutsche's 553bn euros, 52%.
So deep have been Deutsche's woes that for most of the past three years its five-year credit-default-swap spreads—
the cost of insurance against its failure to honour a bond—have been wider, often by half a percentage point or more, than those of the smaller bank.
Last month it sold bonds at steepish yields.
Although a merged bank would have more heft at home,
it is hard to see what a takeover could do to restore Deutsche's fortunes as a global investment bank.
Having wisely given up its own international investment-banking ambitions, Commerzbank has little to offer.
Admittedly, Deutsche is not alone: since 2012, notes a new report by Morgan Stanley,
European investment banks have lost nine percentage points of market share in America,
while Wall Street firms have gained the same amount in Europe.
Though Deutsche is still Europe's biggest, as well as its homeland's flag-bearer, it will need more than a merger to patch its tattered standard.