When too many particles exist at once, newer particles stop updating their size based on the camera position and zoom.
They correctly change size based on the ParticleArray size values, but will no longer take into account where they are being viewed from.

I'm assuming there is an engine per-frame threshold on the number of particles that will update their size, starting with the oldest.
This threshold will change depending on the particle quality.
From the example below, that threshold seems to be about 50 particles for High (which seems extremely low if this a performance optimisation).

Found when adding a permanent particle effect that emits a lot of particles over a long period of time.