By STELLA YIFAN XIE (The Wall Street Journal) It handled more payments last year than Mastercard, controls the world's largest money-market fund and has made loans to tens of millions of people. Its online payments platform completed more than $8 trillion of transactions last year – the equivalent of more than twice Germany's gross domestic product. Ant Financial Services Group, founded by Chinese billionaire Jack Ma, has become the world's biggest financial-technology firm, driving innovations that let people use their phones for buying insurance as easily as groceries, enabling millions to go weeks at a time without using physical cash. That success is also putting a target on the company's back. China, even more than the US, is now under pressure to reckon with the disruptive power of a financial-technology giant. China's banks complain Ant siphons away their deposits, causing them to pay higher interest rates, and is a factor leading them to close branches and ATMs. One commentator at a state-owned television channel described Ant's huge money-market fund as "a vampire sucking blood from banks." Chinese authorities, clearly increasingly uncomfortable about Ant's scale, have started to put limits on the activities it can pursue. Earlier this year, China's central bank undermined a years-long effort by Ant to build a national credit-scoring system. The bank effectively prevented Ant's system from being used by institutions making loans. Regulators have issued rules requiring large money-market funds to sharply reduce holdings of assets that allow them to pay high interest rates. They have pressured Ant to slow inflows into its giant money fund. The authorities are also weighing whether to designate Ant a financial holding company and require it to meet bank-style capital requirements, people familiar with the matter say. That would likely affect its profits, which last year came to $2 billion, pretax, on roughly $10 billion in revenue. Investors remain spellbound, rewarding privately held Ant in June with a $150-billion valuation on paper, more than twice its valuation in a 2016 funding round and above that of Goldman Sachs Group, Inc. For years, Chinese authorities "turned a blind eye and let them grow as large as they could," said Zhu Ning, deputy director of the National Institute of Financial Research at Tsinghua University, who says he has talked with regulators about risks to the financial system posed by Ant. "It is simply incredible that such a gigantic financial institution has slipped away from a comprehensive regulatory framework," Mr. Zhu said. The vice governor of China's central bank, without specifying a company, recently warned that some influential payment institutions shouldn't think of themselves as "too big to be regulated." The central bank didn't respond to requests for comment.