On Android, we ship all the locales in one package (since the Android Market doesn't have a simple way for us to deliver different packages to different locales). That accounts for part of the difference in size.
We're working on various ways to reduce the size, including moving the locale files into separate files that are downloaded at installation time.
Thank you for the answer. Please implement this quickly, I want to recommend FF to pretty much everybody and a huge honkin download is not a good sign on a mobile phone.
We're working on various ways to reduce the size, including moving the locale files into separate files that are downloaded at installation time.