copywriting

Yes, you should just limit your explanation to their requirement. As an interviewer, I am asking this question not to get the full picture of the project, but simply to understand enough to ask you follow-up questions. That's the real point of this question: to see whether you can explain the "why did you do x instead. It's in those follow-up questions that you get to show that you really understood the full project. If you try to show all your understanding up-front, you will get bored looks, as you have already encountered. Also, explaining it up-front only shows that you've been able to memorize a script, not that you really understood the project.