Oh how I'm glad to see it's not "only me, once more" !!
I just upgraded to Leopard, and while moving my data from backup to machine, I notice wildly different folder sizes.
First of all let me mention that since Mac Day One (january 2007), I've always used the inspector to deal with sizes (how could Command+I on a bunch of files ever be useful when it simply opens a corresponding myriad of info windows, try that with 100 files, LOL).
Right now the inspector says my System folder is 290MB. Then I go down one level, believe it or not, inside there is only one folder "Library". And this bloody lonely folder is... 3.41GB !!
Same when the directory contains multiple files. Command+A shows the correct size, but up one level and the size is incorrect.
Even funnier, I went to (root):Library and clicked on "Address Book Plug-ins". Inspector says it's calculating the size. I browse the contents and... surprise ! it's empty !
Selecting all items on the root, I see 135GB. But the disk shows 137.9GB used ! how are we supposed to work reliably ?
I don't recall having such a problem with Tiger. Or with Windows XP (cough cough).
Damn... they add a nifty path bar and quick view, but break the size feature.