I am at work at moment, but I will try to answer your question.
My experience is, that a bunch of materials (many used material slots in blender) also increase the vertex count. I try to put much as possible of the materials on less as possible UV maps and export them to Substance Painter https://www.allegorithmic.com/products/substance-painter. With Substance Painter, its easy to use masks on layers to seperate materials on a single texture. And btw, texturing with SP is so much easier and makes fun, instead of using blender for this work.
I also delete every single not used vertex in my mesh by hand. Not sure this is a common practice, but it also reduce the vertex count.
I think, if you have a mesh model with - let me say 1K vertices in blender with 5 materials and behind each material 5 textures for color, AO, etc, plus the physics hull, HF somehow multiplicate the vertices when you have uploaded in HF. If you have 15 materials, it explode the vertex count anymore.
Okay, this are my experiences, I never found some documentation about this, and also never heard something about that topic of the devs.
Hope that helps Adrian, good luck