It is not only the progress in imaging over that period. Voyager was a more ambitious and expensive mission in general. The mass of Pioneer 11 was 259 kg, while that of Voyager was 825.5. That extra mass included a proper camera with multiple lenses on a steerable platform. This is different from the Pioneer spacecraft, which were spin-stabilized. The scanning that Russell Borogove's answer mentions was actually based on the rotation of the spacecraft sweeping the detectors past the planet. That wasn't because that was the only way they knew to do it - they could have sent a better camera, but there was a budget they needed to meet. With Voyager, they had both better technology and more money.
Really, though, this should be no surprise. According to a quick Google search, the first (American, apparently) cell phones with cameras came out in 2002, thirty years after the first Pioneer launch. The first iPhone came out in 2007, thirty years after Voyager. If you compare the pictures you get from phones at both ends of that span, I think the difference is at least as great.