Hyperbole. Understatement. Irony.
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…