Could also be automatic video compression. Because the iphone 4s has such a low res front facing camera they may able to get around video compression. Compression causes artifacts, as camera pixel density goes up the quality of compression software must also go up to compensate. In this case apple appears to have a solid setup.
Download vs upload speed, in this case upload is just as important but most of the time its never advertised. Upload is typically 1-2/10ths the theoretical download max. In order to upload streaming video and not have frames dropped/stutters the phone must actively convert higher resolution video on the fly to a much lower bitrate while keeping as much of the origonal quality. Similiar to how mp3's are conversions of original studio quality formats. Cant remember what these file formats are called. Anyhow, this conversion is done on the fly before being uploaded. The reason to do this conversion is to lower the data usage. Lower quality = lower data usage. In this case apple just does it better. But you gotta remember, they design a phone then design software specifically for that phone. Samsung and the others (besides google) dont have this luxury, yet.
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