OK, this is my final answer:

perl -pe'
  s!\d{4,}(?=\s)!
    sprintf$"x($x=-4+length$&)
           ."%3.*f"
           .qw(M G T)[$x/3],
      $x%3<1,
      $&/1024**int$x/3+1
   !ge
'
[download]

which is 104 chars not counting the decorative white space.

  p