I don't know if this is really a bug, but it just confused me for a few moments and may be worth bearing in mind .... I was using the Finder (10.2.6) to view my Movies folder, which contains several rendered DV files. One 700mb file is in a subfolder. I was using List View (Command-2) and had used the grey arrow tool to expand the subfolder's contents. I then issued a Select All command and ran Get Info to check the total size of the files.
The reported size was far larger than it should have been. The problem was that Get Info had included the subfolder's size twice: once for the folder icon and once for the icon of the .mov file within. It appears that Get Info totals every file visible in List View, regardless of folder structure. Further subfolders compound the problem.
Here's how to see the problem for yourself. In List View, create folder1, and then create folder2 within that. Expand both, and place a 1mb file in folder2. Run Select All, and then do a Get Info, and the file size will be 3mb -- 1mb for folder1, 1mb for folder2, and 1mb for the actual file.
There are, of course, better ways to discover a folder's total size (running Get Info on the folder, for example), but I often use Select All - Get Info as it's an easy combo to execute from the keyboard when you're working in Finder. So I figure it's worth mentioning.
[robg adds: I confirmed this on my machine by using the inspector window -- Shift-Command-I. Open the inspector, and then use the Command key to select each of the three items (two folders, one file), and you can see the reported size change as each item is added! Seems like a bug to me.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030820164414763