It is best to size the height of the rectangular main duct larger than the round branch duct so that the spin in fitting can be properly installed. This is even more important if conical taps rather than straight taps, but applies either way.
When Revit sizes the ductwork, it will make the height of the main duct equal to the diameter of the branch duct. Since this will not work in the real world, is there a quick fix or do I have to resize all ducts by hand?