The latter is a gentler way of saying, "no, I want
your solution to this trivial problem".
The former is a question about your work process; for instance, if they say, "ok, so build Twitter" and you respond rationally with, "what of Twitter should I build?", they're probably looking for how you operate in an environment with underspecified work. It might be a good idea to talk about how you'd defend your decisions to draw the boundaries of the problem; why you chose to ignore e.g. streaming updates or following; &c.