The first-round draft pick was only active for three games during 2012, and failed to make a catch for the NFC champions.
The wide receiver told the Sacramento Bee: "Obviously, I want to contribute a lot more than I have been. So I'm going to make that happen and come back bigger, faster, stronger.
"I'm going to accountable this off-season and make myself the kind of player that they want me to be.
"Me and [Kaepernick] already are great friends off the field. So hopefully, we can get that chemistry on the field."
The 49ers could find themselves short of wide receivers for the 2013 season, with Randy Moss and Ted Ginn Jr both entering free agency, and Mario Manningham and Kyle Williams recovering from knee ligament injuries.