Gas giants often rotate very rapidly, and this rotation causes them to become oblate. The degree of oblateness is dependent on the both the speed of rotation and the surface gravity, so that the most oblate gas giants are those with relatively low gravity and fast rotation.
Behemoth, a Hyperthermal Jovian, compared in size to Jupiter.
The largest known gas giants are members of the HyperthermalJovian type, since their high temperature causes them to become less dense and this reduces their surface gravity. Cooler gas giants with the same mass are generally smaller, and due to compression they are rarely much larger than Jupiter. The least dense cool giants have very small rocky cores, a subtype that is quite rare - most of these giants are very old, and have lost nearly all the heat of formation that younger examples retain.
Most gas giants have some sort of ring system, although in many cases these rings are almost invisible. Some giants have spectacular rings, often striated due to tidal interactions with the moons of that world.