Why recording a still image using a mobile camera is taking up more space?
A still image doesn't have any temporal redundancy to exploit - it is a snapshot of a single point of time! Also, the quantization parameter will be low. Video compression uses higher quantization parameters since each individual frame is seen for a fraction of a second, so in a normal playback mode, the user doesn't get to focus on the details within each frame. Both of these contribute to the lower file sizes for the average image in a video.