I am testing on a VM as I write and to my surprise I find the XFCE version of Mint to be bigger (by about 1 GB) than both the standard Mint and Ubuntu 8.10. I have only installed what's on the CD ISO and run updates. No extra packages added.
I have investigated a little and I think my original observation is caused by the inner workings of VirtualBox. I was judging the size of the installation by looking at the size of the HDs of the virtual machines as seen by the host system. I have now looked at disk usage from inside the guest systems. I am looking at Ubuntu 8.10, Linux Mint 6 XFCE and CrunchBang Linux. All three are up-to-date and I have *tried* to add the same set of extra packages. It turns out all three are pretty similar in size at around 3 GB of system and applications.
Why the virtual HDs differ as much as they do I don't know. Maybe because I re-used an old one for my Mint test.