Hi grandeson,
Welcome to the site … and I’m glad you found us 
The density of a polyfoam or memory foam material is expressed as its weight per unit volume and in North America the most common way of describing density is lbs per cubic foot (in Europe kg per cubic meter is more common). Mattresses that are larger sizes have a greater volume so they would weigh more overall even though the density of the materials are the same. Lead for example has a much higher density than feathers so it will weigh more with the same volume of material. With polyfoam and memory foam the density of the material is the most important factor that determines its durability because lower density polyfoams and memory foams are less durable and will soften and break down more quickly than higher density versions of the same material. When a manufacturer buys foam for their mattresses the density they specify when they order is one of the main criteria that also determines cost so higher density and more durable versions of a particular material tend to be more costly than lower density and less durable versions of the same material.
When you sleep on a mattress the upper layers of foam will compress and deflect more than the deeper layers partly because the comfort layers are usually made to be softer than the deeper transition and support layers of a mattress (and firmness/softness is also a factor in the durability of a material) and partly because they are closer to the sleeping surface and subject to direct compression without any layers above them absorbing some of the compression forces first. It’s this constant deflection of the foam materials in the upper layers of the mattress that softens and breaks down the foam and leads to the loss of comfort and support in the mattress. This will also happen more under the heavier parts of the body such as the hips/pelvis than the lighter parts of the body. This is why the quality/durability of the upper layers (the top 3" to 6" of the mattress) are especially important in the durability and useful life of the mattress as a whole.
If you are in a higher weight range then you will compress the foam layers that are closer to the top of the mattress more than someone that is in a lighter weight range so those that are in higher weight ranges generally need more durable materials (higher density) than those that are in lower weight ranges for a mattress to last about the same length of time. They would also compress the deeper layers more than someone who was lighter so the durability and density of the top 6" may become a more important factor than it would for those who are in lighter weight ranges (or children) where the durability of the top 3" or so may be the most important because they don’t compress the deeper layers as much.
A mattress is only as durable as its weakest link no matter how much higher quality/density materials it may contain. If there is only about an inch or so of lower quality and less durable materials in the top layers of a mattress that are subject to constant compression then the foam softening of that layer would have very little effect on the mattress “as a whole” but once you reach about 2" or more of lower quality materials in a mattress that are subject to constant compression and deflection then the foam softening of a thicker layer would have a bigger effect on the mattress as a whole and could lead to the premature loss of comfort and support and the need to replace the mattress too quickly relative to the price you paid. This is the reason for the caution about “no more than about an inch or so” of lower quality materials in the comfort layers of a mattress so that too much lower quality/density materials don’t become a “weak link” in the mattress. In an ideal world you want “more” of the good stuff and “less” of the bad stuff in a mattress.
There is also more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
The better options or possibilities I’m aware of in the Monroe/Newark, NJ. region (subject to the quality/value guidelines here) are listed in post #7 here.
While the Morgongava may be a little on the firm side for many people I would keep in mind that Dunlop latex comes in a wide range of firmness levels that range from relatively soft to firm. Firmness itself is also relative to the person and a mattress that feels firm for some people can feel soft for others depending on their weight, their preferences, and what they are used to sleeping on and are using as a reference point for firmness/softness.
Phoenix