그렇게 넣으면
1280x720 이 1280x480 이렇게 출력 되는거 아닌가요 ?

http://ffmpeg.org/ffmpeg-filters.html#scale-1

=> If one and only one of the values is -n with n >= 1, the scale filter will use a value that maintains the aspect ratio of the input image, calculated from the other specified dimension. After that it will, however, make sure that the calculated dimension is divisible by n and adjust the value if necessary.