First of all, let me say that the viewer in Imatch is very good!!
When I hold down the left mouse button, it zooms in. But sometimes it zooms to 100% and sometimes a lot less. Why? I know I can use the z-key to toogle between 100% and full view.
Is it possible to set it to toggle between for example full view, 25% zoom and 100% zoom?
Holding down the left mouse button does nothing.
Alt+leftclick and moving the mouse left/right zooms between 25% and 800%.
See Zooming (https://www.photools.com/help/imatch/#viewer.htm?dl=h-16) for more info.
I have seen this several times that holding down the left mouse butten does zoom in. But perhaps some settings are wrong. But of course it works the solution you mention.
But is it possible to toggle between several zooming levels using the z-key? (full view, 25% zoom in and 100% zoom for example)?
All zoom methods, mouse and keyboard, are explained in the Viewer help topic I've linked to above.
If what you want is not mentioned there, it does not exist.
But there are more than plenty of methods to zoom so you should find something that works for you.
Most likely 'frlindla' is referring to lef-click --> dive zoom
This toggles between fit-to-window and 100%.