Hi lotus65,
That sounds like it was damaged during shipping or storage from being compressed and is defective.
With two toppers that are the same ILD (a measurement of the firmness level of the material) a thicker topper will be softer than a thinner topper. Thickness and firmness level both affect the softness of the topper. Softer toppers are more pressure relieving but more risky for alignment (they can allow the heavier parts of your body to sink down too far relative to the rest of your body which can put your spine out of alignment) while thinner toppers will keep you closer to the firmer support layers in your mattress but will be less pressure relieving. Different materials also compress differently and become firmer at different rates as they compress more deeply. There is more about the different specs that can affect softness and about the balance between the conflicting needs of pressure relief and support in post #4 here.
The key with adding a topper is that it is “just enough” in terms of thickness and softness (in combination with the softer foam that is already in the top layers of your mattress) to provide good pressure relief in your most pressure prone sleeping positions so the risk of alignment issues is lower.
Phoenix