As a candidate, I feel that I should ask the interviewer if they're seeking a simple solution or a very scalable one. In this way, I can try to tune the response to the specific interviewer's expectations.
You're backing into things like QPS, size of the data you're responsible for hosting, uptime requirements, real-time requirements, write load vs read load.
Often the natural walk is "let's assume it's a low load, design for that, and then we'll get to higher load at the end". Other times, it's an actual systems-y problem they're facing as a company, and that they have a putative solution to compare your knowledge against.
But yeah it is generally really important to codify and at least state (if not explicitly clarify) your assumptions before recommending a solution.