Hello. I am entering 9 months post FUE transplant (1800 grafts for a norwood class 2). Thank you

The reality is that there is a physiologic limit to how close you can place the individual follicles in any one procedure.  This has to do with the blood supply to the individual grafts.  The hairs are normally fed by many tiny arteries, which are by definition all cut during extraction.  When the grafts are placed into the recipient area, it takes 5 or 6 days for the blood supply to return.  During this critical time, the grafts survive on a passive process of diffusion.  It simply sponges up nutrients that are always sitting underneath the skin.  This clearly does not supply as much blood and nutrients as an active blood supply, so each graft requires a small cuff of undisturbed scalp around it to literally feed each graft for the first few days until the blood supply arrives.  As such, a certain percentage of people, particularly those with thin hair shaft diameters, will desire a second procedure to increase density into the transplanted area.

Not sure how you are able to judge the difference in diameters.  You may be observing that the transplanted grafts are growing "thinner" than expected.  It may be a perception or expectation issue - a subjective observation rather than an objective observation.  Follow up with your doctor and measure the diameter with a micro-caliper. 

Usually the majority of hair begins growing in the first 6 months post-transplant, and the hair caliber improves the second 6 months post-transplant.  You should begin seeing an improvement in hair caliber soon.

in some rare cases. Also, one transplant usually wont get you to 100% density so its important to have your expectations in line with the possibilities and discuss this in detail with your doctor.  Another thing is if they split the grafts.  Some places split and trim grafts to single hair follicles and that changes the way results look. 

To increase hair diameter finasteride and topical minoxidil could be used. PRP injections could also be performed in the area.

Most people are paying attention to the harvesting technique as oppose to the technique used in the recipient area.  A hair transplant specialist should know how to dense pack the areas of concerns without compromising the blood supply, by maneuvering the size of the grafts and also the overall design.  That being said, due to hair cycle the transplanted hair usually starts to come in and in some cases may take up to 18 months for full growth.  So at this point, I suggest for you to be patient, wait and few more months and see if your growth is satisfactory at that time.