the perception might be that caricature artists know what all the features of the face are and then when they see people, they are attuned to which dimensions stand out as being unusual and they are able to point these idiosyncratic proportions out in their drawing by exaggerating how far --at least several of the more notably prodigal dimensions have been offset by the hand of god or nature,  but the truth is that those dimensions--those proportions themselves become features to the artist over time. a beginning caricature artist will set out with some knowledge of common features and proportions and then examine faces in much the way that most would think. they effectively classify all of the major features and "exaggerate" the idiosyncrasies. the idiosyncrasies being aspects of a given face which lie outside of all of the artist's normal facial feature types.  so then if someone's nose is too big to fit the mold so to speak, the artist makes the nose size bigger because that's the one thing that disqualified it from conforming to the set. but eventually when the same deviations keep popping up repeatedly, they become features in and of themselves.