He is 1-1 with a 2.22 ERA in four starts with Iowa after going 0-2 with a 6.48 ERA after four appearances with the Giants.
"He's progressed," Cubs general manager Jim Hendry said. "He's not quite where he was when he was doing well in Frisco, but we feel he's close. He's been working hard, and his velocity is working its way up. He's in better shape than when we got him.
"He's worked very hard and done everything we asked him. I don't know actually how much weight he actually lost. But he's certainly in better shape than when he arrived. But he's very honest and upfront about it. He's not passing the buck on anybody. And he's pitched pretty well the last two or three starts."
Williams was 1-1 with a 2.22 ERA for Iowa in four starts. He had a 3.30 ERA for the Giants in 2003 and went 10-7 with a 4.25 ERA in 2004.
Williams is slated to face Milwaukee on Tuesday and the White Sox on Sunday.