An oxymoron is any 2 words used together that contradict each other. For years people have used "shrimp" as a means of describing things that are small. In that light using the word "jumbo" with it makes it an oxymoron, describing something small as being big. Just like "Reliable iPhone" is an oxymoron.
First off, what IS an oxymoron? "Bittersweet" is an oxymoron because those are opposite flavors. "Deafening silence" is any oxymoron because silence can't literally be deafening.
1.There is no reason a shrimp can't be jumbo. There are small shrimp, and there are big shrimp.
2.You can call a person a shrimp to indicate they are small, but when would you ever call a person a "jumbo shrimp"? Never! The only situation you would call a shrimp "jumbo" is when you're talking about a decapod crustacean. The only time the word jumbo is paired to shrimp, is when the word shrimp isn't indicating the size of something.
3.If you can call a tall person a "giraffe", would "tiny giraffe" be an oxymoron? I would think not.
In closing, the idea that jumbo shrimp is an oxymoron seems like a joke among English professors that got out of hand and became a serious part of the English language. (Kind of like how the Schrödinger's cat thought experiment was initially a critique of the absurdity of the Copenhagen interpretation of quantum mechanics, but has come to be widely cited as an explanation of it).