I think what is at play here is simply the difference between a dynamic and condenser mic. A condenser mic is indeed more sensitive than a dynamic and will take to softer/gentler passages better. The condenser should be better able to pick up nuance, but is easily overdriven. Singing louder is where the dynamic would be starting to shine.

Of course singers "work" the mic differently too. With the condenser, I get the singer to stand further back if they are going to project a lot. With the dynamic (I use an SM57 here), you sometimes need to be very close, and also very steady or you risk dropout (but which can be used to good effect for song dynamics with a trained vocalist).