During the spawn is a good time to practice you finesse fishing. Scaling down the patterns and slowing down the retrieves can be the only way to catch them during the daylight hours. I have had the surprising success with sz 10-12 leech patterns designed for bluegill and trout. There are somethings you have to consider though when fishing light gear.
Your chances to actually land a big bass
Getting a bass to hit a small pattern is relatively easy, but landing them is a whole different matter. Smaller flies are tied on lighter hooks, many of which are barbless. It is very easy for a big bass to shake free. Make sure you are using quality hooks and keep tension on them throughout the fight. Don’t sweat it if they get off. It’s part of the game.
Water clarity
You need relatively clear water to fish smaller patterns. Dark or off color water significantly reduces your chances. Target structure with a bow and arrow cast if in tight quarters. Let the fly sink before you start your retrieve.
Depth
Presenting a small fly can be a challenge. If the bass are shallow it’s fairly straight forward, but many fish are holding at 10-20 ft. You should try a variety of presentations. I recommend preparing a sink tip, float and fly set up, and mono rig (most commonly used for tight lining trout).
By Catch
When you are using smaller patterns, you often have to weed through a multitude of smaller fish. Bluegill and small bass are voracious eaters too. Don’t change it up though- stick with it and you will get into some bigger fish.
Ending thoughts
On heavily pressured lakes, small flies are often the way to go. I usually rig up two rods- one heavy (10ft 6wt) and one light (10ft 4wt). Throughout the day I am switching between the too. It takes some getting used to and the is usually fly line everywhere. Going light can salvage a day of fishing when things conditions seem tough.