DIFF defines areas of silicon into which various dopants will be diffused in order to build up the MOS or BiPolar or whatever. Areas that are not DIFF are isolation areas usually a thick oxide insulator. These are typically 0.25um deep for 0.13um to 0.40um deep for a high voltage BCD process. any dopant that gets into the insulating layer will be electrically inactive and thus irrelevant. Only doping that goes into the DIFF will become usable.
The reason NIMP and PIMP is larger that DIFF is to allow mis-registration between these implant layers and DIFF. Under no circumstances must any area of DIFF that is to get doping, not get it because the IMP mask is misaligned to DIFF. By oversizing these masks relative to DIFF, this source of error is eliminated.