Hi abbysdad,
Not necessarily no. The upper layers of a mattress are the most important when it comes to the density and quality of the materials and without knowing the specific details of the layer thicknesses and the density of the materials in each mattress there really isn’t a way to meaningfully compare them.
For example … memory foam has a higher density than polyfoam so you could have one mattress that used 2.5" of poor quality and lower density memory foam over a higher quality polyfoam base layer and another mattress may use higher quality memory foam over a lower quality base layer.
Some math to illustrate the point …
Mattress #1:
If the top layer was 2.5 " of 3 lb memory foam would weigh (in queen size) 20.8 lbs.
If the middle layer was 2.5" of 1.2 lb polyfoam it would weigh 8.3 lbs
If the bottom layer was 7" of 2.2 lb polyfoam it would weigh 42.8 lbs.
The total foam weight would be 71.9
This mattress would be very risky in terms of durability because the top layers are very low quality/density and would soften or impress more quickly than higher quality materials.
Mattress #2:
If the top layer was 2.5" of 4 lb memory foam it would weigh 27.8 lbs
If the middle layer was 2.5" of 1.8 lb polyfoam it would weigh 12.5 lbs
If the bottom layer was 7" of 1.5 lb polyfoam it would weigh 29.2 lbs.
The total foam weight would be less at 69.5 lbs
This mattress would be much less risky because the top layers which are the “weak link” of most mattresses are higher quality and would resist softening and impressions much better than the first mattress (which I wouldn’t even consider) even though it is lighter overall.
It all depends on which layers are higher quality and density not only on the total weight of the mattress.
Having said that the odds are “tilted” towards the mattress at Sams Club being higher quality although I personally would never buy a mattress where I didn’t know the details of the layers because they could both have “weak links” and the quality and value of the mattresses at Costco isn’t as good as many other choices that you have available to you (some of the better memory foam choices are listed in post #12 here for example). The only “good” thing about buying a mattress from a big box store is that you get a free “roll of the dice” because of their refund policy.
Phoenix