Method 1: Fixed size scale with padding:
ffmpeg -i inputfile.mov -filter_complex 'scale=578:462, pad=720:576:71:57' outputfile.mp4
This assumes SD PAL size input and output. This simply uses a fixed size pad.
Method 2: Percentage scaling with overlay on top of black generated by filter:
ffmpeg -y -i inputfile.mov -f lavfi -i color=c=black:s=1920x1080 \
-filter_complex "[0:v]scale=w=0.80*iw:h=0.80*ih[scaled]; \
-map "[out]" -map "[aud]" \
-strict -2 \
This assumes input and output size to be full HD (1920x1080). The scaling is by 80 percent. So the overlay position is 20 percent inside- but since this 20 is divided on both sides equally, the overlay uses 10 percent of main width and adds that to x position.
eof_action is required so that when the video file ends processing can stop. Else the generated black (background) from
-f lavfi will just keep on going.