When ripping movies from iso to full side by side format, 3840x1080 usually (1920x1080 each eye). Shouldn't the bitrate double? I've always wondered that, what do you think?
Bitrate isn't linearly proportional to frame size. As frame size increases, it becomes more efficient to compress each uniform-sized block of the picture*. Consider that Youtube recommends that users encode a 720p video at 5mbps, and a 1080p video, containing 2.25 times as many pixels, at 8 mbps - 1.6 times the bitrate.
Also, in case of a stereo image, the two pictures have to be very similar to each other, in order to produce a coherent 3D image perception, so there's a lot of redundancy to be exploited by the encoder.
*if there's a lot of fine detail in the image, then it is not as efficient.