The term “giant planet” is deceptive. The outer four planets are mostly gas and liquid or ice, possibly over a much small solid core. But they can likely only maintain this size while in orbit around the Sun.
Thrown out of the solar system, any number of things might happen to them that would diminish their size, leaving them looking much like a gigantic comet.
And then we see them on "Behind the Music" after they're all washed up....talking about how they had it all, were the biggest planet, etc...