detailed means having many details
brief means short
So how come are they antonyms?
what if something is detailed and still short even after providing all the details? (it totally depends on the topic)
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Don't compare a document on one topic with a document on another instead consider two documents on the same topic. Consider the famous book By Stephen Hawking: "A Brief History of Time" it is 226 pages long, but is still brief when one considers how much was omitted!
Suppose you write a brief document. In order to achieve brevity you presumably choose not to say some things. What name would you give to those things that are omitted? We could say that we are leaving out some detailed points, or we are omitting details.
If we are asked for more information we would produce a more detailed document. The detailed document on the subject would be less brief.