Property(ies) that make memory foam soft or firm?

What would be the property or properties that would the same volume of gel memory foam softer or firmer? Would that be density?
I’ve tried googling this, and even looking on this site, but it’s really not clear.
The differences that I see in the ‘soft’ memory foam mattresses is simply that layer(s) of memory foam are thicker. That is not what I am asking or looking for.
I want to know, if you have a 2" layer of foam, what makes one harder to squeeze with your hand than another 2" layer?
My son has a MF topper, and I ordered the exact same one about 9 months later. However, the new one - same company, same product, everything - is noticeably firmer (read: harder to compress). Does gel-infused foam soften with age?
I want the softer/squishier version, but I don’t know how to find it because I don’t know what properties to look for.