I had a revelation today while I was zoning out.
I figured out the reason why vessels grow and what causes it to happen.
The vessels were created of god and void, that's literally just stated by The Pale King. What matters here is the god part of the statement. Specifically The White Lady.
The White Lady is a tree, a plant, whatever. That means that her children are ALSO partially a plant (alongside being partially wyrm). Plants grow when exposed to LIGHT.
This explains why The Hollow Knight is the largest vessel out of every single one because they were CONSTANTLY around a light source (PK). It also explains why all the other vessels in the abyss and other spots are much smaller, since much like a plant, their growth is stunted if not exposed to light.
The big objector to this theory is of course, broken vessel. However I believe that their exposure to The Infection / The Radiance (aka another light source) explains their growth. It could also explain their uneven horns, which is not present in other vessels.
Anyways boom yeah game theory thanks for watching.