HOUSTON (Reuters) - The Federal Reserve's $4.5 trillion balance sheet and its decision to keep interest rates only just above zero are not delivering as much stimulus to the U.S. economy as might be thought, a top Fed official said on Friday.

That is because headwinds like an aging population are holding back U.S. and global growth, Dallas Fed chief Robert Kaplan told a Texas Oil and Gas Association meeting in Houston. "We don't think the economy is overheating," he said. "We are not as accommodative as people would think."

(Reporting by Ann Saphir; Editing by Meredith Mazzilli)