Litotes is understatement whereby you convey what you mean by saying the opposite...as when you call the 6'10" basketball player "Shorty" or the two-month-old infant "Tiger."
"How lovely!" she thumped,
As she glanced at the dump,
"So fragrant, so neat and so--nice.
I hadn't realized before
That they had a free store
And a luxury home for the mice."
Oxymoron occurs when two apparently contradictory terms are brought together to form a sharper perception. You know all about it--why falling in love is "bitter-sweet."
"Run slowly," she warned,
"Or you'll fall up the hill,
And you'll make a nice mess of your face!"
So like a wise fool,
I rushed snail-like to school,
Displaying rhinoceros grace.
(From a handout I got in 6th grade English class many years ago.)
If you mean relatively or comparatively tall, that's a different matter...