Under certain circumstances, Civil 3D 2017 can cause the DWG file size to grow with each save. If the user does not notice this, it can eventually render the drawing difficult or impossible to open. The file size increase is related to data shortcut surfaces, and although there is no confirmation, we suspect it is related to the new option to store a copy of the surface in the drawing.
As of this writing, there is no official fix from Autodesk delivered to the public. Apparently Autodesk is sending individual fixes to certain customers, per this forum post, which does include a fix and instructions for application.