There are several plugins for reducing image size exists on the web, for example, EWWW Image Optimizer, WP Smush.it and so on. They all have different features and allow you to find the configuration you need. We recommend using the EWWW Image Optimizer plugin.
Once the plugin is installed and activated, you need to go to Media > Bulk Optimize to optimize your images:
You will have an option to bulk optimize all images in Media library using Start Optimizing option. Here you can also optimize all other images outside Media library such as graphical elements of the theme and so on with Scan and optimize option:
NOTE 1: The optimization procedure requires considerable resources so hitting the CPU resource limit temporarily is probable.
NOTE 2: The plugin allows determining a degree of image compression. For example, if you own a photo blog, note that well-compressed pictures will lose quality.
You can also set the plugin to conduct scheduled optimization in Advanced Settings.
Scheduled optimization does not include Media Library images, because they are either already optimized or will be optimized on upload.