Takes the output from du -k - file block count in 1024-byte (1-Kbyte) blocks and converts to a human readable form.

convertToHuman.sh

# $1 = file block count in 1024-byte (1-Kbyte) blocks.
# i.e. output from du -k
# no floating points, just ints, but check anyway
function convertToHuman {
    size=$(echo $1 | sed -e 's/^\./0./')   # add 0 for cases like ".0001" 
    size=${size/\.*}                       # convert to int
    for unit in KB MB GB TB PB EB ZB YB; do
      if [ "$size" -lt 1024 ]; then
        echo "${size} ${unit}";
        break;
      fi;
      size=$((size+512)); # to round to nearest you add (denom/2) to the numerator
      size=$((size/1024));
    done;
}

printReport.sh

function printReport {

  local startSize="${1:-0}"
  local endSize="${2:-0}"
  local savings=$(echo "(1 - ($endSize / $startSize)) * 100" | bc -l | sed -e 's/^\./0./' )

  # if the file size is over ~100 MB, start using the human output 
  if [ "$startSize" -gt 100000 ]; then
    startSize=$(convertToHuman "$startSize")
    endSize=$(convertToHuman "$endSize")
  else
    startSize=$(printf "%'d KB" "$startSize")
    endSize=$(printf "%'d KB" "$endSize")
  fi

  if [ ${savings/\.*} -gt 0 ]; then
    printf "Optimised %s to %s (saving %2.2f%%)\n" "$startSize" "$endSize" "$savings"
  else
    printf "No savings. Start: %s End: %s\n" "$startSize" "$endSize" 
  fi

}

#### NOTES

printf "%'d" "$NUMBER" 2> /dev/null
 
# prints out integers with thousand separators. The dev/null redirect is to handle floats.
 
# e.g.
 
# printf "%'d" 1034343.56 2> /dev/null
# 1,034,343
 
# printf "%'d" 1034 2> /dev/null
# 1,034

Takes the output from du -k - file block count in 1024-byte (1-Kbyte) blocks and converts to a human readable form. e.g.

james@Jamess-iMac: ~/Projects/Github 
$ du -sk 
2141672 . 
$ convertToHuman 2141672 
2 GB 

Main issue - It rounds down as bash maths is integer only. Still it's a start ...