What is the rhetorical term for magnifying the impact of a statement by precedin...

Here are several examples of this rhetorical device.

1. You aren’t pretty—you’re beautiful.
2. I’m not angry—I’m furious.
3. He received a certificate for participating in the race—and a medal for taking first place. (Here, the first part is seemingly a contradiction of the second one because no one would mention a certificate if they won a medal.)

This rhetorical device is called correctio or epanorthosis, where a speaker corrects a weak descriptor with a stronger one.

OED has for epanorthosis, "A figure in which a word is recalled, in order to substitute a more correct or stronger term." It doesn't list correctio, but that is merely a Latin word for "correction".

I'm not convinced that (3) is an entirely apposite example. The first two definitely are.