The Securities and Exchange Commission proposed sharply raising the size threshold of funds required to report their U.S. stockholdings quarterly, a move that would end such disclosures for nearly 90% of current filers, including many hedge funds and mutual-fund firms. The proposal would mean investment managers with less than $3.5 billion in such holdings—including securities bought with leverage—no longer need to reveal the details of their portfolios. Those details include the number of...