Michael, your experience with FP4+ and HP5+ in PMK is perfectly normal. Fine grained films do stain less than courser grained films. And from the description of your procedure, you did everything correctly.(posted 8447 days ago)As for the grain of HP5+ and FP4+ being equal, I am suspect. PMK does show fine grain with HP5+, due to the staining. But PMK also yields very fine grain with FP4+. In my experience, FP4+ grain is significantly finer than HP5+ when developed in PMK.
I wonder if you are comparing equal density negatives. If the FP4+ negs are somewhat overexposed, that could account for the grain increase. Also, I think I use 9 min for FP4+, and, like you, 13 min. for HP5+. I don't have my notes handy, but it might be that the FP4+ negatives are a little overdeveloped.
It might also be something as simple as comparing an FP4+ image which has an expanse of out of focus midtones to an overall sharp HP5+ image. All things being equal, FP4+ should show significantly finer grain.