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.