Bear in mind that I know less about trout-fishing than most people on this board, but I did study ichthyology in general at the graduate level. So here's my .02...
People do studies all the time of what fish prefer to eat, and in general (with room for lots of exceptions), the larger the fish, the larger their preferred prey. Once I netted a bunch of juvenile bluefish about 3-4" long for a study, and in the lab they regurtitated lots of tiny copepods that they had been feeding on. Obviously when they get just a little bit longer they turn to a fish diet. You can even see this, though, with fish like eels, which in freshwater start out eating insect larvae and the like but incorporate more and more small fish into their diet as they get larger.
With that said, I think that it is good advice that you have to match the hatch, because even very large fish will eat small food if it is super-abundant. Think of a bear eating blueberries. The bear might prefer to eat salmon, but if there aren't any salmon around, it will go with the abundant food. Fish are the same way.