Thumb doubles the image file size

What you need to do is to compress the image. WordPress offers a filter to set the compression ratio.

function my_jpg_compression($args) {
    return 75;
}
add_filter('jpeg_quality', 'my_jpg_compression');

NOTE: What you are returning is the compression ratio. 70-80% is
manageable and you will not notice much reduced image quality. Also,
after applying this filter only future image’s size can be reduced and
not the one which are already uploaded.

What for already uploaded images?

You can use some awesome plugins to reduce image size and still keeping manageable quality. One of such a plugin is WP SmushIt. In fact you can use this plugin for future uploaded images too.