It's only megabyte heavy if you use all of Dart's reflection capabilities. If you don't use dart:mirrors, Dart is competitive in size when compiled to JavaScript, especially when you consider that a modern web app today contains a bunch of support libraries like jQuery (which itself is ~30kb). With Dart, you don't need jQuery because Dart's dart:html and dart:core libraries are pretty good out of the box. Of course, do your own measurements as each app is different.