XnView Software

XnViewMP 1.9.8
Windows11 (25H2), display resolution 3840x2160, dpi scale 150%

Viewer - fullscreen

to reproduce:
0. clean portable version
1. set start with file to 'Viewer - Fullscreen'
2. set Autozoom

• Normal mode
  any, except 'None' (and 'Last used' if previous was 'None'; by default its set to 'Best fit'),
  eg 'Fit image to window'

• Fullscreen mode
  No Fit

3. open any image
image will be very small (~50x50px)






Related

• issues
  1.9.8 - viewer - auto zoom - normal mode affects fullscreen (current post)
  - todo -
  1.9.3 - viewer - auto zoom - 'Fit image to desktop' does not fit
  1.5.5 - viewer - pan - issues with selection/zoom/fullscreen
  - fixed -
  1.8.3 - viewer - auto zoom - resets to 'No fit' (1.9.4)
  1.7.1 - viewer - auto zoom - fit image to desktop issues (1.7.2)
  1.4.4 - viewer - auto zoom - mode inherits from fullscreen (1.4.5)

• suggestion
  - done -
  Viewer - auto zoom - swap 'Fit window to image' and 'Fit image to desktop' (1.7.2)

Thanks to your detailed description I can reproduce the problem.