You can try the following if you would like to overlay a video onto a master (background) video, without re-encoding.

ffmpeg.exe -i master_video.mp4 -vf “movie=smaller_inner_video.mp4[inner];[in][inner] overlay=70:70 [out]” completed.mp4

where 70:70 in this example are the positions that you would like the smaller video to be overlaid onto the master.