How to lossless optimize images with plugins for grunt-contrib-imagemin?

I am trying to lossless optimize JPEG / PNG images with imagemin, but I have a problem when using extensions, they "deoptimize", meaning the result image is bigger than the original. How can I prevent this?

This is my Gruntfile.js

...
grunt.initConfig({
    imagemin: {
        jpeg: {
            options: {
                //by default it uses imageminJpegtran
                progressive: true,//default false, progressive JPEG is better compression https://superuser.com/questions/463477/progressive-jpeg-quality-compare-to-normal-jpeg
                arithmetic: false,//true breaks image

                //don't know how to use it loseless without making the images even bigger
                // use: [
                //  imageminMozjpeg({
                //      quality: 95, //Compression quality, in range 0 (worst) to 100 (perfect).
                //      dcScanOpt: 2,//2 Optimize between one scan for all components and one scan for 1st component plus one scan for remaining components
                //      arithmetic: false// false, or it breaks the image
                //  }),
                // ],
            },
            files: [{
                expand: true,
                cwd: '/www',
                src: ['**/*.{jpg,jpeg,JPG,JPEG}'],
                dest: '/www',
                filter: 'isFile'
            }]
        }
    }
});
...

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