Pioneer 10 and 11 launched in 1972 and 1973, respectively, whereas both Voyager probes launched in 1977. I do expect the imaging quality to improve in 5 years but the difference is a bit too much.
Pioneer 11 image of Jupiter:
Voyager 2 image of Jupiter:
Pioneer 11 image of Saturn:
Voyager 2 image of Saturn:
The images were obviously processed to show more detail and make the images clearer and brighter, but still, the difference is stark. Why are the Voyager images so much better than the Pioneer ones?