In what situation would a pipe's overall length decrease?

The scenario in which a pipe's overall length would decrease is when a fitting is over-tightened. Tightening a fitting too much can distort the pipe's ends, causing them to compress and effectively reduce the length of the pipe. This excessive force can lead to mechanical deformation of the joint, resulting in a shorter overall length as the fitting draws the pipe ends closer together.

On the other hand, reaming a pipe is intended to smooth the inner surface and does not typically affect the external length of the pipe. Cutting threads involves removing material to create a threaded section, which changes the ends of the pipe but does not result in an overall decrease in length. Filling a pipe with solder generally increases its weight but does not impact its length. Therefore, the correct answer emphasizes the mechanical consequences of over-tightening fittings, which can lead to a reduction in pipe length.