Take this 3D-Printable spool holder for instance. In the picture above it appears to have come out far smaller than it should. 25.4 times smaller to be exact, which can happen...
It's possible that the STL was exported in the wrong units! STL's need to be exported in millimeters, but many CAD programs (at least in the US) default to inches. Read more about STL exporting here.
There are two ways to solve this problem. Of course, it's best if you can get a hold of the same STL file in the right units. But if you can't do so, there is a clever feature in KISSlicer that solves this problem for you:
Right-arrow click on the icon representing the file, and click "Inch -> mm" to scale the file up by a factor of 25.4. It's a really quick fix to a potentially annoying problem. Boom, all done!