This Revit Recess entry will explain the behaviour and characteristics of insulation and lining. The keyword here is Connectors. The image below is of isolated round duct sections where no insulation or lining has been applied yet.
By default, the connectors attached to the T-Piece is the same size as the duct attached to it. The 25mm Insulation is highlighted in Red, with 25mm Lining highlighted in Green
The image below shows the relationship of the duct connector to the geometry of the T-Piece in the family environment.
What if the connector diameters should change in the family itself, with the geometry staying as is? How would that affect insulation and lining applied to the fittings and its respective ducting in the project environment?
After making the changes above and loading this revised T-Piece in the project, we can now clearly see, especially around the T-Piece, that the Insulation and Lining is added from the MEP connector, and not from the geometry edges.
Have fun!