Lots of unionists seem to see in the problems of Bank of Scotland (HBOS) and Royal Bank of Scotland (RBS) the ultimate proof that Scotland is too small to be independent.
Not all unionists are convinced, however.
Apart from Fraser's points, it's worth remembering that independence would have consequences in many unexpected ways, and the size of banks is likely to be one of them.
If Scotland had been independent for the past hundred years, it's unlikely that BoS and RBS would have grown so big. Other small countries don't tend to have any banks that large. (Apart from Iceland, that is.)
It might also be that independent Scottish competition authorities would feel obliged to split up the banks as they're too dominant here.
But even if Scotland was home to the headquarters of massive financial institutions, they probably would have separate structures in Scotland and England, and Scotland would only have to save the Scottish bit.