Maybe because less pixels in windowed mode?
I don't know, but I've been keeping track of the Fusion - Parallels war for some time now. I've used both, and it seems to me that Fusion is no faster than Parallels for the tasks I require... it's no more stable (not to call either of them
unstable) than Parallels... and, I've found, it seems to use more resources than Parallels.
I do have to agree that trying to run Parallels or Fusion with only 1GB of RAM is just not going to please... even if Parallels/Fusion is the only application running. I would suggest a minimum of 2GB, and keeping your Windows virtual machine at a lowly 640 to 768MB of RAM.
With all that being said, here comes the shameless plug for the new virtualization software of my choice: VirtualBox from Sun. While it still has a ways to go in terms of feature-for-feature comparison with the other two, it rocks my world and seems to have a smaller memory and processor footprint than the others while still maintaining a level of performance that is above acceptable and on-par with both Parallels and Fusion. It doesn't, at the moment, support drag-and-drop between host and VM, "shared folders" are weird to use (you have to perform a "connect network drive" or access the shared folder as a network share), and its handling of using ISO images as mounted optical media is a bit clunky... but it works for me just fine, and best of all, it was MUCH kinder on my wallet (read: it's FREE!).