Larger image size in outdoor lighting for H.264

Question

I recently did an experiment where I recorded H.264 compressed video on a Samsung Galaxy Nexus running Android 4.1 under different lighting conditions--office lighting, dim lighting, and bright outdoor lighting. For a 4Mbps bitrate encoding, the average frame sizes across the different lighting conditions were all similar to one another. However, at 500Kbps bitrate encoding, the average frame sizes in the outdoor lighting condition were much larger than those in the office and dim lighting conditions; these latter two were similar in size. For 56Kbps, the dim lighting had the smallest average frame sizes, then the office lighting, and finally the outdoor lighting was again the largest on average. Is this a consequence of H.264 encoding, the Galaxy Nexus' camera, or what?

Answer 1

First, I'm not sure the question fits the guidelines of the site, so you probably won't get much of a response.

That said, I don't think it's a matter of the camera. The better lit outdoor setting will simply have more color detail, making compression more difficult. Using a higher bitrate to encode likely loosens the reigns on the compression process so the resultant frames are more similar in size.