Ghostscript doesn't generate form XObjects from PostScript forms. The form XObjects are also flattened during PDF to PDF conversion. This limitation has been known before but a Ghostscript user Benjamin Waechter <benj at waechter dot org> has posted a real world file on comp.lang.postscript that we convert to 17M PDF but Distiller 5 converts to 970K .
Created attachment 1264 [details] Mailing anniversaire Hippopotamus2.ps.bz2
Bug #687561 "Smaller PDFs when using execform" specifically adresses this problem. It also need, for part of its functinality (like forms with XUIDs that are loaded into VM more than once), bug #687560 "Invalid PDF if /BP pdfmarks with non-unique /_objdef". Unfortunately, GS has some other bugs in handling pdfmarks that create Form XObjects, for example bug #687558 "Wrong clipping path with /SP pdfmark" (the subject doesn't tell the whole story) and at least another one that I'm aware of but haven't analyzed yet. Both seem to affect the attached sample.
*** This bug has been marked as a duplicate of 687561 ***