Rhetorical device I can't find the name for

Is there a word for the rhetorical device where the reader is meant to imagine the something in order to make the reader think that it is worse or best or whatever than words can generally describe? It’s like when someone won’t tell you something, so you assume the worst, and then when whatever it was comes out, it’s almost never that bad. Instead of real life, though, it’s a deliberate rhetorical device. Examples: “Think of the most beautiful thing you’ve ever seen. It was more beautiful than that,” or “If you can think it, he’s done it, and probably worse.”

Many thanks!

Hyperbole?

Hyperbole sounds like it’s getting there; I know I’ve seen this in Roman writings, but can’t remember where, and I don’t have my Quintilian handy, so good luck.

Best source I’ve found to look up rhetorical stuff is Silva Rhetoricae: The Forest of Rhetoric

It might be amplication by comparison.