The tab "Extensions" shows information on size grouped by file types. Thus, you'll have an overview of the types of files that use up most of the disk space. Use the "Limit to this Extension" option from within the context menu to only show sizes of the selected type in the File System Tree and the Details. Use "Show all Files of this Extension" to list all files of the selected extension.
By clicking on adequate icons in the toolbar of the tab you can save this list in a file, print or copy it. Another option available in the toolbar is to show a small stacked bar chart on the bottom of this tab, indicating all extensions space usage.
Use the function "Group similar file types in one group" form the tool bar to show similar file types together into groups like "Audio Files", "Video Files" or "System Files".
You may configure the file groups in the Options dialog (See: "Options > File Groups").
Note: Within an update scan, the size information shown in the "Extensions" list and in the directory tree could differ.
After the scan is completed, the shown information will be consistent again.