Hi @Jacques,
Currently “BIM” elements sent from C3D are being sent as generic Speckle pipes - this means they do not contain Revit specific information like families and Revit parameters. The message you are getting lets you know that the pipes that were created in Revit were assigned to a default pipe family, in this case Hydronic Supply, which comes with a default diameter.
We’re constantly improving our Revit conversions, and will look into adding diameter adjustments to the default conversion. In the meantime, if you’d like to adjust your Revit pipe diameters to match the civil pipes without doing it manually, then the easiest way is probably through the Dynamo connector, in which case you can pull the diameter information from your Speckle pipe and generate a Revit pipe with the correct parameter value!