Personally, I've given up on commercial lines and for about three years now I make my own. It's extremely thin and pound for pound, the strongest line available. You can too. Here's now.
Catch four, or eight large spiders depending on if you want 4 or 8 strand braid. You know those large black spiders that scoot across the floor and startle you. Go to the department store and purchase the largest twist drill you can find and some superglue. Now, carefully glue the belly of the spiders equidistant around the end of the drill and put it in your drill chuck. Glue them good as centrifugal force is a ***** once it gets going.
Now the fun begins. Go down to your local high school football field. Touch each spider's abdomens to the goal post so that they start a spinneret strand. Make sure you are pointed up field. Slowly walk away and run the drill (hopefully you realized you need a battery drill right?). Try to keep the drill turning and your backward walking just fast enough so that the spiders can keep up with their web strand production.
The natural stickiness on the strand that catches flies will adhere the strands together and by the time you reach the far goal line you'll have 300 yards of some of the strongest strands found in nature. Using a sharp knife cut the line and backtrack by winding it up on a spool after it dries. In humid areas drying time might take a few more minutes.
For the cost of a drill and a little of your time you've got a 300 yard spool of 4 or 8 strand fishing line that might have cost you $35 if it had been purchased from a tackle store, or $18 on ebay. And the best thing is, you can do it again.
Another note: If you plan on making this a one off thing, please safely return the little critters to the wild. After all, they did you a big favor and there are mosquitos out there so they can keep helping us out.
If you plan on keeping them for future line making, you'll need a good supply of flies to get you through the winter. Just put a couple on your windowsill. They breed like crazy, especially if you've got an uncooperative dog, and you'll have plenty.
Oh.... put the spiders in separate jars. They eat each other....
I hope this tutorial helps those out there who are concerned that their fishing lines aren't a consistent thickness. It's very important.