ognjen.kuljic

Some very useful plugins like Regenerate Thumbnails leaves old files behind, so it is good practice to clean after it.
Features: 
*Remove all unused files from upload dir (Unattached files to posts or pages), 
*Remove all leftover files (forgotten files in upload dir).

qCleanup

This plugin allows you to delete unused and leftover files from upload dir. In one click you can rid of all unwanted files and reduce space usage.

Jason LeMahieu (MadtownLems)

University of Wisconsin - Cooperative Extension

This plugin adds a column to the Media Library page that displays the total space used by each media item. For images, this includes the original image as well as any other generated sizes.
This plugin is particularly handy for networks where site owners have a limited space quota.
Additionally, sorting your Media Library by file size is actually a great way to identify duplicate media items.

Media File Sizes

I REALLY needed this, to be able to identify the largest images, and I'm glad to see it still works in April of 2018

Still Works

Easily see how much disk space (in KB, MB, & a percentage of your quota) each of your uploaded files takes up.

qCleanup vs Media File Sizes