BIG streamers = BIG fish, right? That's very true......but not always.
Sometimes downsizing is a good thing to do. Big streamers certainly have their time and place, but so do little ones. In fact, my biggest trout ever came from Pyramid Lake, NV on a tiny #8 streamer that was no more than 2 inches long, and I've also caught my largest snook on a similarly-sized offering. A versatile fly angler won't get stuck on fishing one size fly all the time, but will select the proper fly size according to the given species and conditions he or she faces.
Why might a smaller streamer be a good choice?
Water Conditions: A small streamer can be a great choice when water conditions make the fish more wary. Gin-clear and very shallow water would be a great example of this, especially combined with calm wind and a clear sky. While the fish are more easily-spooked in conditions like these, the increased visibility and a "shorter" water column allows smaller, more subtle flies to be spotted and intercepted more easily. I have seen many instances where a big fly is just "too much" in waters like these and the fish will show no interest or even get scared by the presence of a large fly.
Pressured fish: When I'm conventional bass fishing on a popular body of water, one thing I know when fishing these often highly-pressured waters is that downsizing can sometimes work wonders. The same holds true when fly fishing, too. On heavily-fished waters where the fish see much of the same stuff day in and day out, using a smaller and more realistic pattern is sometimes a good tactic to try. Just like in conventional bass fishing, you can think of it as a kind of "finesse fishing." In addition, a smaller fly may hit the water with less commotion and be less likely to spook nervous fish nearby.
Match the Hatch: Here's an obvious one—not all forage fish are the same size! If the predominant baitfish in your body of water are tiny but you're tossing a noticeably larger fly, you're likely putting yourself at a disadvantage because the fish are likely keying-in on that smaller size. Big fish don't always eat big meals!
Less Impact: One nice thing about a downsized hook is that it has less impact on the fish. With its smaller gap and thinner wire, a smaller hook won't poke such a big hole in the mouth of a fish and has less chance of inflicting more severe damage.
Easier to Cast: Depending on the weight and materials used, a smaller fly may be much easier to cast. Fishing that smaller pattern may even allow you to drop your setup down a weight or two. I personally always love fishing smaller line weights the most both for the casting and increased fish-fighting pleasure. If desired, you can also fish a lighter leader since a lighter fly rod will protect it a little more efficiently. Dropping down the leader size also lets the fly act a bit more natural and sink a touch faster.