My usual recommendation for viewing the sizes of various folders, or just to find where all your space is going:
Sequoia View
Granted, it's not Windows Explorer, but it's something.
Having Explorer check all the files in each folder to check for filesize would slow it down considerably - it would need to scan your entire drive every time it was opened. I hate just waiting for Explorer to spin up my optical drives every damn time I open Explorer, even if I haven't opened them. It still insists on checking them - and it really gets me if I'm about to perform a write operation, like decompressing files, and it still looks at my read-only DVD drive.
