First of all, currently there is no "Shrink bitmap" mode, we have two fit options instead. Apparently "shrink" means the "Fit image to window" mode.Furthermore, pictures can be printed too. In the mode 'Shrink bitmap' the picture is shrunk to the size of the available print area, and also enlarged, if it was smaller. Otherwise it is printed in 1:1 size.
Now about printed size:
From my test (Print to XPS in Windows 8.1, format A4 portrait) with default Lister settings, large pictures are always resized to paper size, no matter whether one of two Fit modes is selected or not. Enlarging a small picture happens only for "Fit image to window" mode when the picture is also shown enlarged on the Lister's screen. In "Fit only larger images to window" mode the small pictures are printed 1:1 like without a fit.
Finally, any user-selected zoom does not affect printing at all.
It would be reasonable to improve the help section about printing to reflect existing rules.