SpaceSniffer is a freeWare (donations are welcome) and portable tool application that lets you understand how folders and files are structured on your disks.
By using a Treemap visualization layout, you have immediate perception of where big folders and files are placed on your devices. Treemap concept was invented by Prof. Ben Shneiderman, who kindly permitted the use of his concept into this tool. Start a scan process and see the overall situation. Bigger are the elements on the view, bigger are folders and files on your disk. You need more detail on a big folder? Just single click on it. The selected element will be detailed with its content. Need more and more detail? Keep selecting elements and they'll discover their secrets to you. Need a larger view of a small folder? Then double click on it, and it'll zoom to the full extent of the view.