Phosphorus does affect flowers, but it does not make flowers bigger necessarily. It will NEVER make non scented flowers into scented one.
Back to the original question, smaller flowers can occur when they are rebloom on the older spike.
I have to disagree with the energy thing.
I would think growing a entirely new spike with bigger flowers will take more energy than making flowers on the spike that is already present. It is just my reasoning and I like it.
I have only seen one phal out of so many over the years, make smaller than "normal" sized flowers on older spike.
My opinion is not energy thing ( I actually do not understand what people mean when they use the word energy to describe plants physiology), but rather simple physics.
Smaller flowers mean less weight than larger flowers of the same flower count, thus the spike is less likely to break off and lose all those flowers.
Remember that flowers are reproductive organs of plants and they need to make sure flowers have to stay on until they finish their "mission".
When my phal rebloom from the old spikes, they still makes flowers of the same usual size.
The one that makes smaller ones is the plant that makes lots of flowers. Think of this. There is this tall skinny spike from the previous season. Then a branching spike develops and make some more flowers. As the flowers develop, the weight adds on and risk breaking the spike off.
Smaller flowers, thus less weight, will solve the problems. Just a thought.
I really do not like the idea of the "energy" thing.
If a plant has enough "energy" to make flowers, it will. Plants know what they are doing given the right conditions and proper care.
---------- Post added at 11:27 PM ---------- Previous post was at 11:23 PM ----------
Regarding the slight color change, seeing it as less red tone than more yellow would be more accurate, because phal hybrids (and some other flowering plants as well) with red tone will lose that when bloomed under warmer temperature.
That's just how red pigmentation works.
It intensifies under cooler conditions. It has to do with sugar production and storage under cooler temperature.
I would say this change on yours is very subtle. I've seen some big change in many, and I did not like it. lol