bloat.rb

gems = Hash.new(0)
gems_size = Hash.new(0)
total = 0

`du -ks ~/.rvm/gems/*/gems/*`.each_line do |line|
  size = line[/\d+/].to_i
  name = line[/\/([^\/\n]+)$/, 1]
  
  gems[name] += 1
  gems_size[name] = size
  total += size
end

bloats = 0

length = gems.keys.map { |a| a.length }.sort.last

gems.each do |name, num|
  size = gems_size[name]
  bloat = (num - 1) * size
  bloats += bloat
  
  puts "#{num} of #{name}".ljust(length + 5) + "Size: #{size}kB"
end

puts
puts "    Total gems:  #{total / 1024}MB"
puts "    Total bloat: #{bloats / 1024}MB"
puts "    #{100*bloats/total}% of your gems are duplicates."

“No such file or directory” when I run this in OS X

Well, it only checks if you have duplicate gems in your RVM installation, so if you don't use RVM it won't work.