Figures of over-statement and under-statement
Figures of over-statement are done for effect, to emphasize a point you are looking to make, and include:
Hyperbole-Use of exaggerated terms for emphasis or effect.
His eloquence could split rocks
Auxesis-Reference to something with a name disproportionately greater than its nature (a kind of hyberbole).
The wound given to my client... (when referring to a scratch)
Litotes-Understatement used deliberately.
Running a marathon in under two hours is no small accomplishment.
It isn't very serious. I have this tiny little tumor on the brain. —J.D. Salinger, The Catcher in the Rye
Meiosis -Reference to something with a name disproportionately lesser than its nature (a kind of litotes).
The scratch my client gave to the plaintiff... (when referring to a sizeable wound)