USP is more so about the unique mechanism by which the product works (and no other product can replicate). Whereas the Big Idea can extend beyond the product & into the zeitgeist.

For example:

If I’m selling protein powder, my USP might be about my product is made with grass-fed dairy, which is easier to digest.

But my Big Idea might be that you are a victim of the food-industrial complex, whose obsession with cost-cutting has led to a generation of bloated, weak men… whereas my product will reunite you with the healing power of natural, organic protein.

In other words, the USP/unique mechanism is the intellectual justification for purchasing, but the Big Idea is the eфmotion that makes the audience want a way to justify purchasing.