Hi AJHODGE,
The thickness of a mattress or any individual layers inside it is really just a side effect of the design and the design goals of a mattress and is also only one of many variables that can affect the feel and performance of a mattress relative to any particular person and by itself isn’t particularly meaningful (see post #2 here). In some cases higher weight ranges will sometimes do better with a mattress that is thicker than lower weight ranges (see post #14 here for more about the effect of thickness) but even this depends more on the specific design and combination of materials in the mattress and on how well your testing or personal experience indicates the mattress “as a whole” matches your specific needs and preferences in terms of PPP than anything else.
The density of memory foam and polyfoam isn’t directly related to firmness and any density of memory foam or polyfoam can be chemically formulated to have a fairly wide range of firmness levels and other properties (such as resilience, point elasticity) as well. Every layer and component of a mattress will also affect the feel and performance and firmness level of every other layer and component in the mattress and the mattress “as a whole” and there are also many variables that can affect the firmness of a mattress besides the firmness of one of the layers inside it (see post #4 here).
Density is the most important factor in the durability of memory foam or polyfoam but it has much less to do with how it “feels” and you can’t feel the quality or durability of a material when you sleep on it.
Latex is a little bit different and the density of latex is much more closely related to it’s firmness level and if you are comparing the same type and blend of latex then higher densities will be firmer (although this doesn’t necessarily apply when you are comparing different types and blends of latex because Talalay is less dense than Dunlop in the same firmness level and synthetic latex is also less dense than natural latex in the same firmness level as well.
Again with everything that you can “feel” (as opposed to the durability of the materials) the best way to know whether any mattress is a good “match” for you in terms of “comfort”, firmness, and PPP will be based on your own careful testing and/or personal experience.
Phoenix