One of the oddities of all version of Windows Explorer is that there is no easy way of finding out what deep directories are stuffed with fat files and taking up all your disk space.
That's where tools like Treesize Pro, an excellent shareware tool comes in. We've been using it for years through it's different version and really like it. But it does cost and when doing a many hundred machine roll out an Open Source / Freeware alternative can save one a heap of cash.
So if you need such a beast have a look at WinDirStat which is fast and does more or less the same job. And it's free.