How much bigger would a large U.S. bank's balance sheet look if it had to follow the same accounting rules that European banks do? We're starting to get a peek.

Of the six largest U.S. banks, so far only Citigroup Inc. and Morgan Stanley have filed their first-quarter reports with regulators. Citigroup showed $1.88 trillion of assets as of March 31 using U.S. accounting standards, which let companies show "net" figures on their balance sheets for derivatives and certain other types of financial instruments.

That number would have been $2.86 trillion if Citigroup used gross figures for the same items, which is what the International Accounting Standards Board's rules require. The difference between the gross and net figures stems largely from so-called netting agreements that allow U.S. banks to offset huge chunks of their assets and liabilities against each other for financial-reporting purposes.

The bigger numbers are available as a result of new footnote disclosures required by the U.S. Financial Accounting Standards Board. While the boards may still have different standards, at least now outsiders will have an easier time making apples-to-apples comparisons between the financial statements at, say, Citigroup and Germany's Deutsche Bank AG.

Here's a simple example: Using U.S. generally accepted accounting principles, Citigroup's total assets are about 10 times its $190.2 billion of common shareholder equity. If you use the gross figures from the bank's footnotes instead, that ratio jumps to 15, which makes Citigroup look more leveraged. Deutsche Bank showed 2.03 trillion euros ($2.6 trillion) of total assets, or about 36 times equity, as of March 31.

As for Morgan Stanley, it showed total assets of $801.4 billion on its March 31 balance sheet, which was about 13 times equity. The gross amount was $1.75 trillion, or about 29 times equity.

(Jonathan Weil is a columnist for Bloomberg View. Follow him on Twitter.)