MPEG(H.264, H.265) compression is based(among other things) on image correlation between frames. More noise => less correlation between frames => less compression.
Big sensors => small pixel size => more noise(especially at night) => less correlation between frames => less compression.
But still surprised 8MP did so much different at night. Looks like such cameras frequently should be used with a lower resolution during the night. If let's say 4 pixels downscaled to 1(which I believe camera simply does if the smaller image size is requested, it's denoise spatial filter by itself ).
I wonder how your 8mp cameras would behave at night if used as 2MP camera. And are the results consistent across cameras in this case?
Not sure about the numbers. I just do not know physics of sensors enough, maybe you are right. I think it also would depend on which noise reduction filters are are currently used on cameras. For this reason, I interested IPVM to test it.
Great article. Your findings correlate well with what we have identified during our day and night testing with 1080p cameras and 8MP cameras. In a site with several hundred cameras, we always see a handful of 8MP cameras that increase their nighttime bandwidth well beyond the other cameras (20mbps to 100mbps). These cameras typically have much more digital noise caused by a number of factors including dirty lenses, IR illuminator energy reflecting off of adjacent structures, IR illuminator energy from other adjacent cameras illuminating the problem camera, street and building lights shining into the cameras, floating debris in the air (dust and dirt), and insects flying around the cameras.