If I am laying out a plantroom at say, 1:50 scale, I would normally draw the larger pipework (maybe everything above 100 dia?) to show its full size, ie, 3 lines (including the two 'sides' and the centreline) . As for the small stuff, well clearly there's no point in drawing these three lines in a 25 dia pipe because they would merge as one heavy line!
The decision as to what point to revert from a single line to full width, is very much dependant on the eventual scale the drawing is intended to be plotted at, and is generally made at the discretion of the draftsman / designer.
Revit doesn't appear to give the operator that discretion.....does this mean we are going to end up with ugly thick lines representing small diameter pipework, and other such issues everywhere?
(And I'm not sure that the answer to this one can be found by changing the detail levels.....)