Since you are using Inventor 2009 (three releases old) you do not have the Shrinkwrap tool that will automatically simplify the assembly. Instead you will have to open a new part, exit the sketch and then perform a Derive. Locate the assembly, and then start removing parts that the Microstation guys don't need to see. That will start reducing the overall file size once you do save it out as a Step file or DWG solids which will then be brought into the DGN format. Keep in mind all they really need to see is the extents and where the pipes should hook up to in your assembly. They really shouldn't care about anything else.