Compress video files recursively, using x264, placing the output files into the same folder with an appropriate filename (filename_scaled.mp4). First argument is the top-level directory to scan. It avoid overriding already compressed files. Change the filetype (e.g mp4) at the beginning of the script.

compress-mp4.sh

#!/bin/bash

FILETYPE=mp4

if [ "$#" -ne 1 ]; then
    echo "Usage: <path-to-compress.mp4> target-top-level-directory"
    exit
fi

# for all the mp4 except the already scaled ones
find $1 -type f -name "*.$FILETYPE" -not -path "*_scaled*" -exec sh -c '
for file do
    dir=$(dirname "${file}")
    filename=$(basename -- "${file}")
    extension="${filename##*.}"
    destfile="${dir}/${filename%.*}_scaled.${extension}"
    
    # -n will prevent overriding the destfile if exists
    # The lower the crf value the higher the quality. DVD quality is crf 25
    # You can use -threads to limit the number of threads used.
    ffmpeg -i "${file}" -vcodec libx264 -acodec copy -crf 27 "${destfile}" -n
  done
' sh {} +

# TO DELETE the original files do
# find path -iname "*.mp4" -not -path "*_scaled.mp4" -delete