Grunt plugin: responsive_images - files sie are too large

Question

I'm using https://github.com/andismith/grunt-responsive-images for image resisting. But the outcome fillies size is too big. For example the original file size is 277KB with dimensions 2880x1800, and the resided file has size of 533KB with dimensions 1000x625.

These are the settings of the plugin:

 photo: {
                options: {
                    engine: 'im',
                    sizes: [{
                        name: '1000',
                        width: 1000,
                    }]
                },
                files: [{
                    expand: true,
                    cwd: '<%= config.app %>/assets/images/posts',
                    src: ['**/*.{png,jpg,jpeg,gif}', '!**/*-{600,900,1200}*.{png,jpg,jpeg,gif}'],
                    dest: '<%= config.app %>/assets/images/posts'
                }]

Can somebody explain why is this happening, and how to fix it.

Thanks