There is no precise technical relation between pixels and bitrate. For the same streaming provider; higher the pixels, higher the bitrate and vice-versa. Different service providers can offer different pixels even at the same bitrate. If pixel quality is fixed, then the number of pixels in a video is fixed. E.g a 1080 p will have 1080*1920 = 2073600 pixels. But how much of data/video size that has is not determined directly by number of pixels. Thus, Youtube, Vimeo, and Vdocipher can provide different bitrate/size of videos for the same length and same pixel quality.