Original and Progress Topo

Hi Bo

Welcome to the forums. 

Without seeing the project it is a bit difficult to diagnose exactly what the issue is, although I would guess that what is happening is that you have used the automatic outline when creating the existing feature surface around your 1200 surveyed points. 

It sounds like your proposed surface created with your drone survey is then larger than this existing surface.  Cut\Fill will not be calculated in areas where there is proposed and no existing so it is important to ensure there is enough existing to calculate against.  Instead of using the automatic outline on your existing survey surface which uses a 'shrink wrap' algorithm I would suggest using an extrapolate outline and manually tracing an area larger enough to encompass your proposed.   The below image demonstrates the issue, in both examples the proposed is the same, but in the second example there is not enough existing surface to calculate against so the cut\fill is very different.  This of course can be beneficial in some circumstances if you want to restrict the calculation only to a specific area (although using volume regions would be superior in most circumstances but it is still and effective technique which is used).

   

Let me know if this isn't the issue.   If you have screenshots you can upload them into a post.  It's a tricky you need to upload and then click 'add attachment' and then you can insert it into the post.  This video also might be helpful as it discusses the different outline types of a feature surface.  The automatic outline actually creates a 'varying levels' outline by default.  

Understanding Feature Surface Outlines