When do we say "You little liar" and "You big liar"?
Do the words "little" and "big" mean what they mean?
"You little liar" mean "You tell small and not so serious lies"
and
"You big liar" mean "You tell big and serious lies"
Or "little liar" and "big liar" just mean "tell lies in general and we don't know if the lies are small or big"
Or the words "little" and "big" are there for emphasis only and not actually tell us if the lie is small or big and "You big liar" is stronger "You little liar"?
Best Answer
The word "little" here is native colloquial English, in the UK at least, but this specific example is likely to be of, to, and between children, or in a child-like manner.
The little here, is to emphasise the offense perceived, and diminish the person it is addressed to. Similar phrases you might hear in arguments between adults - "You little bastard". "You little thieving shit". "She's a little cow". "You're the little snitch who told the police, aren't you?"
In all of these, "little" is used to belittle the person referenced, and emphasise the feelings of, and word used, by the speaker. It adds a feeling of distain and perhaps disgust or contempt, of the other person being weak, small or otherwise worth mocking and disregarding.
But perhaps because adults less often shout things like "you liar", that's more likely to be something said by, or related to, children, and therefore "You little liar" has a slightly childish petulant feeling that "You little bastard" (for.example) does not.
Update:
See also the Wikipedia article on intensifiers in swearing (AKA expletive attributives), which is almost exactly how 'little" is being used here - note the comment that "Words that are never thought of as offensive can be used in similar ways".