This is a part of my "better defaults for KDE software" initiative.
Because Gwenview has the {nav Enlarge Smaller Images} setting off by default, two out of the three image zoom mode buttons appear to not do anything when you're looking at an image that's smaller than the viewing area. This causes user confusion and makes part of Gwenview seem broken, reducing confidence in the software.
It's true that turning on automatic image enlargement by default will make very small images like raster icons look bad in {nav Fit} and {nav Fit Width} mode when blown up to the full image viewing area size, but that's why there's a {nav 100%} button right next to them! I think we should trust the user here, rather than attempting to protect them from themselves and making two of the zoom modes appear randomly broken for certain images.
Besides, the type of people who typically look at a lot of tiny raster icons are usually software developers or other professionals who are generally more computer-literate than average.