Hi bantrycourt,
Congratulations on your new mattress 
There are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
While in very general terms it’s certainly true that the materials in a mattress or a mattress “as a whole” can feel firmer for those that are in lighter weight ranges than they would for those that are in higher weight ranges. the only way to know whether any mattress is a good match for you in terms of “feel”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) will be based on your own personal experience since each person is unique.
Like all all the simplified choice mattresses the Tuft & Needle has a great return policy that lets you try the mattress in your bedroom instead of a showroom with little risk so the “worst case” is that if the mattress isn’t a “good enough” match for you in terms of PPP and you don’t sleep well on it that you can donate/return the mattress for a refund.
I’m looking forward to your comments and feedback about how you like it once you’ve received it and have had the chance to sleep on it for a bit.
Phoenix