If you look at fly fishing over the course of the last 20 Years you will see a trend in flies, especially Streamers, growing in size and color. A lot of this stems from the advancement of the new fly tying materials that allow us to tie bigger brighter flies and still be able to keep the weight down so we can still cast them. But the question remains, does this help us catch bigger, or more fish?
Thinking about this question, I think you need to break it down by species, which I will for at least a few of them. Also what I am about to write is mostly based on my opinion and I am willing to bet not everyone will agree with it so please if you do not agree with this you will not hurt my feelings.
For Bass (largemouth) I would say it helps us get to the larger fish simply because we are fishing with a fly the size of the fish that would normally snatch your fly if it were smaller. Although this may be true most of the time people do catch large fish on small flies.
(Mini Bass Smasher - Melon)
For Steelhead I would say that flies have grown vastly from the old school Spey flies of the past helping us imitate the food that they are mainly eating when they are still in the ocean like the squid and shrimp colored patterns that are offered on this website. I would also say this has helped people catch more fish based on the amount of pictures I see on social media of people catching large chrome steelhead.
(Drug Money Intruder - Burrberry)
For Rainbow Trout I would say it matters a bit less. Yes, when fishing bigger streamers for rainbows you are targeting the bigger meaner more aggressive fish, but I don’t think you’re going to catch more fish than you would if you were say swinging a small leach or minnow pattern that matches the size of bait fish in the water you’re fishing.
(Pig Sticker - Baby Bow)
For Brown Trout I would say that it helps having a larger flashy fly because most brown trout I have ever caught were very aggressive when they ate my fly. Also In my opinion Brown Trout target bigger prey then rainbows, at least in California. I can’t speak for other areas because I’ve been limited to fishing mostly not far from where I live.
(Articulated Scratch N' Sniff - Brown)
For Pike based on what I see on the Internet I would have to say size really matters. I will say I have never targeted pike but would love to very much. I have many friends that do however target them and most of their flies are usually from 9” to 12” long with loads of movement.
(Chicken On A String)
Ok so if there is one thing I hope you get from reading this is that yes you can fish big streamers and catch big fish but (for most species) you may not catch more fish from fishing larger than normal streamers. It will, however, help you Target the larger more predatory fish that, in my opinion, are more fun to catch.