Windows does not show the folder size in windows explorer by default.Only way to check the actual size of the folder is by looking at the properties of the folder or by hovering the mouse over each folder.This method actually becomes very weird when the number of folders are very large.
A cool utility called folder size solves this problem.This utility adds a new "Folder Size" column to Windows Explorer displaying size of every folder.
Add Folder Size Option in windows explorer( Win XP)
Download Folder Size from here
Run the downloaded file to install Folder Size
Open an Explorer window in Details view.
Right click on the column headers to see a list of columns you can add. Choose Folder Size.
Make this the default view for all folders. Go to Folder Options from the Tools menu. In the View tab, click Apply to All Folders.Now you can see the Folder Size of every folder inside the explorer window itself.