Is there an NFA whose corresponding DFA has lesser number of states than the given NFA? (without unreachable states or epsilon moves)
Consider an automaton with $n$ states, all final, and a transition between every pair of states for every symbol.

There are two variants of the subset construction.  A "formal" one where always $2^n$ subsets are considered, and a "practical" one, where you consider only states that are reachable. Of course only the latter interpretation of subset construction will work.