You can see the total size of the folder if you have tool tip enabled under folder options.
Hold your cursor over a folder and it will show you the full size of that folder and all contents including sub-folders.
But yes, I know what you are talking about, this is a feature missing from XP, that should be brought back.
I am guessing no one thought about it, or they figured no one would be using the old status bar anymore as the new one can be more useful in certain respects. There is some data that is missing that needs to be implemented in my opinion to the new status bar. One of them is what you mention here.
Maybe they can fix it with an update or SP1.
I don't believe this is a bug, but a missing feature, as I do not believe it is in Vista either. I think I noticed that there also, and figured someone would fix it eventually. But, who knows, I am not sure I really miss it that much myself.