A shrimp is not a synonym for smallness, but a name for a marine animal that became a synonym for smallness. Thus, "jumbo shrimp" is not an oxymoron.
"Those cars" is not redundant. It is a prepositionally statement.
But I agree that 'free gift' is a redundancy in a strict sense, although the expression in its entirety is rather moronic. Because there is no such thing as 'non-free gift' it is both redundant and silly to even put 'free' in front of the gift. Affirming something that exists only in affirmative is just as stupid as denying something that doesn't exist (i.e. non-chalant).
Apparently the authors of KJV liked it so much they even put it where the Greek text did not even mention 'charisma.'
Technically, "free gift" is not even a redundancy, as I explained earlier. When I answered, I took you to mean that "free" was a superfluous adjective. In common usage, there is nothing wrong with "jumbo shrimp" as an oxymoron. For a purest, the phrase "bitter-sweet" might do. My favorite example of a redundancy is "born-again Christian". :)