[CentOS] yum update bug with disk requirement calculation?

I just updated my machine from CentOS 5.2 to 5.3 and noticed an oddity. The update required around 280Mb of disk space. I had over 500Mb of free space on root. And... the update failed with "need more space" errors.

Now I can understand this; normally it would fetch the files from a http server, so the worst-case requirements are double (or more) than the download size reported by yum.

EXCEPT these files were being served from an NFS server, and so were never copied local to the machine.

Eventually I manually installed a few smaller packages and managed to get the space test passed. During the transaction I monitored free disk space in another window, and it never went below 300Mb free.

Bizarre.