Culture plays a major part in flower size , i know this from my Phals.I had 1 in my front room all winter & it extended it,s spike.Temps & conditions in my front room are a lot poorer than the growroom & the resulting flowers were very small in comparison to the first 1,s in the growroom.
Maybe that plant is lacking in 1 of it,s cultral needs.Maybe light levels not high enough.
I don,t change fert levels or frequency of fertilising for my Phals & they seem to spike & bloom fairly well in my conditions.