I got annoyed by all that weird window behaviour so i tested them all for once, if they really do what they say. Here's what i found out:
Fit window to image:
What is this setting good for, if you don't enable "Force Xnview to fit image" as well? It seems to do nothing, but display the image at 100%. So shouldn't those 2 settings be merged?
Fit image to desktop:
First up, it's a little slow. The image is first being loaded at 100% zoom, and only then being resized to fit the desktop resolution. That's causing a lot of delay for displaying.
The other problem i got here is that nothing happens if you resize the application window. The image will remain at whatever zoom-ratio it had been assigned in order to fit the desktop. IMO it should behave like the "Fit image to window" setting and resample the image though.
Apart from those problems i'm also missing a "keep ratio" setting. But that's not as important.
Fit window to image is supposed to fit image window to image size independently of size of whole XnView window. Currently not quite working because of problems with MDI.
I cannot confirm loading delay. But if you experience it, vote for final solution for problems with... MDI.
As for behaviour on window resize- maybe it should be a Suggestion. I would not expect automatically changing maginification after image is displayed, unless there is an option that explicitly indicates this.