A heavy person would compress any mattress more than a light person would, so it would definitely feel firmer to a heavy person than it would a light person.
Whaat
Maybe it has to do with mattresses with springs? Like maybe an especially light person wouldn't be heavy enough to overcome their spring force constant and therefore wouldn't sink into it like a heavier person would?
Yeah I saw it as a perception of firmness vs soft. Im light and dont sink into most beds so my perception of the mattress would be that it is firm, and since the heavy person would sink where i would not i would think they see the mattress as soft.
Imagine being the size of a mouse. The mattress would be as solid as the ground. (Specifically the mattress, not the sheets or the padding — those would be soft).
Does it help if you think of it as "floating"?
Like, lighter people "float" more on the mattress, and it feels more solid and firm - heavier people "sink" into the mattress more, and it feels softer.
My first thought was same as Topper Harley (GreyKnight), but on other hand, at least at extreme, mattress conforms much less to lighter person, so I assume less weight distribution and higher percentage of body weight on particular body parts, which will feel firm. 🤷♂️