Hyperbole is a deliberate overstatement or exaggeration of a feature essential to the object. I saw it ten times.
A variant of hyperbole is understatement in which smallness is exaggerated: A woman of pocket size.
Hyperbole is aimed at exaggerating quantity or quality. When it is directed the opposite way, when the size, shape, dimensions, characteristic features of the object are hot overrated, but intentionally underrated, we deal with understatement. I haven't seen you for ages.
As it is mentioned about understatement above, that it is type of hyperbole. But there is another point of view: "Understatement is a form of irony: the ironical contrast inheres in the discrepancy between what one would be expected to say and his actual refusal to say it." (Cleanth Brooks, Fundamentals of Good Writing: A Handbook of Modern Rhetoric. Harcourt, 1950). Michael Jordan was okay at basketball.
Irony is a stylistic device based on the simultaneous realisation of two meanings: the literal meaning is the opposite of the intended meaning; used in ridicule, contempt, or humour. Emphasis is placed on the opposition between the dictionary and the intended meaning of a statement: one thing is said and the opposite is implied. Nice weather isn't it. (On a rainy day)
Intonation plays an important role in expressing irony!
Irony is generally used to convey a negative meaning, but only positive concepts may be used in it (as above: great, nice). Her distaste is impeccable.
:)