Would a larger amount of metal particles increase the conductivity?
Up to a point, yes.
Is the ratio of metal particle to grease really low?
No. Thermal pastes are already >70% solids by volume, and at some point it’d cease to be a paste and would become a putty that doesn’t flow and could lead to air inclusions.
Alright gotcha this makes a lot of sense. My next question then is why are thermal pastes so bad at conducting heat compared to their pure metal counterparts? I mean even the most conductive paste I can find, boron nitride paste, only has a conductivity of a little over 30 W/m.K whereas CBN has 1300 W/m.K.
Heat conduction requires physical contact. Thermal resistance behaves just like electrical resistance: a narrow conductor is a worse conductor of heat or electricity than a wide one, for any given material. So even if your substance is 75% solid, if the physical shapes mean lots of bottlenecks, that increases resistance.
For the sake of illustration, let’s pretend our theoretical paste has metal particles that are little spheres, like ball bearings. The three-dimensional packing density of equal spheres is about 74%. Now look at the contact area between any two spheres: it’s a theoretically infinitesimally small contact patch. That tiny patch is bottleneck, an area of high resistance (thermal or electrical). We have a bunch of spheres, so with many of these contact patches in parallel, in aggregate we do get some useful conductivity, but it’s far, far less than if you took those spheres and melted them down into a block.
I have no idea what the actual particle shapes in thermal greases are, nor what those shapes’ packing behaviors are. But you certainly can’t get anything approaching conductivity in proportion to the solids % in the paste (by which I mean that e.g. 75% aluminum particles in the paste won’t get you anywhere near 75% of the conductivity of solid aluminum.)