While piping up a particularly tight area a person on our team blurted out that the pipe fitting symbology is just too big. This job was being done in Revit MEP in single line.
The answer is to go into the Mechanical Settings dialog by clicking the small 45 degree arrow to the right of the Plumbing and Piping label on the ribbon. Then highlight Pipe Settings on the left and adjust the value of the Pipe Fitting Annotation Size. It's tucked away, but super intuitive once you get there.