I don't think the codec uses 30kbps by default, that specific example is showing what it would look like at 30kbps. Obviously if you have the speed for it, it'll probably raise the bitrate more. Duo is known for adaptively scaling up and down and smoothly switching between different network conditions.

There are four large .gif files. I believe the one you are referring to is 2.6 MB and named `Duo_540X540_TransparentBG_3s_splitscreen_05.gif`, with a split screen comparison of what it would look like at 30kbps. The overhead is the graphics of the "montage", I figure.