Created file with aerial photo for the Layout and added layers with layers showing piping systems. Able to compress Layout photo by 90% with acceptable resolution. However, can’t compress layers. So layer size with some simple lines, lettering, and shapes is about 1 to 1.5 MB.
Are you sure no other pictures are embedded? Copy the file and change the file name extension of the copy to .zip. Open the archive and look into the folder “pictures”. Without picture the file should not be so large.
Or start a new Draw document with same layer structure, but add the “aerial photo” as link and do not embed it. Then copy the other drawing elements. Has the new file such large size too?