The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
to catch a big fish you need big bait.... sure a little 6" brook trout is less likely to take a stab at your size 6 hook than an 18", but that doesn't mean that the 18" doesn't spend most of its day snacking on many of the same tiny morsels that the 6" does.