Fleet of Worlds by Larry Niven and Edward M. Lerner - Inexplicable Gas Giant Obesity
On page 17 paragraph 3, the start of third sentence reads, 'One of its worlds, a gas giant alongside which Jupiter would seem puny...'
This is odd. Gas giants don't get much bigger than Jupiter. They can be much more massive, but they don't actually get that much bigger. This is because as more mass is added to a gas giant its gravity increases and its matter becomes more compressed. We can see an example of this in our own solar system with Jupiter having 3.34 times Saturn's mass but only 1.73 times Saturn's volume. If we increased Jupiter's mass 50 times it would only become something like 10-30% bigger and if we increased its mass 80 times it would become a star, so it's not really possible to have a gas giant that would make Jupiter seem puny, unless perhaps there were some special circumstances. I'll let you know if the book mentions any.
Then again they may have simply meant that Jupiter seemed puny in terms of mass and I am making a mountain out of a molehill and adding mass to it until it undergoes fusion.