when you call a fat guy slim or a tall guy tiny what is the word that this defines

It's irony (noun) - the use of words to convey a meaning that is the opposite of its literal meaning.

Similarly, irony may be a statement or situation where the meaning is contradicted by the appearance or presentation of the idea.

Adjective: ironic or ironical. Also known as eironeia, illusio, and the dry mock.

In literature: a technique of indicating, as through character or plot development, an intention or attitude opposite to that which is actually or ostensibly stated.

Examples of Ironical Statements (from fos.iloveindia.com):

One of the identical twins says to the other, "You're ugly!"

I saw a fish drowning.

Many things can be preserved in alcohol. Dignity is not one of them.

Never argue with a fool. People might not know the difference.

Marriage is the leading cause of divorce

I have been down so long, it looks like up to me.

Tag: obesity 
Thursday, August 24 2017
Source: https://www.thoughtco.com/irony-figure-of-speech-1691196