[praptak]

Both are diminutives. The thing with diminutives is that they can show both affection and disrespect.

It's usually the first if referring to familiar people and the second if referring to strangers. Even exactly the same word can cut both ways. "Dear" is affectionate towards family members, calling a coworker "dear" is condescending.

Also, as other posters have said, rule-based logic can only get you so far. It's all in the context.