There are 2 ways to reduce the quill so that it isn't so bulky.
1. Soak the burnt & dyed feather overnight then ; holding it by the tip grasp the fibers on the side you want to use as close to the tip as you can achieve a good grip, pull the fibers down at a steady rate and force. This should result in the fibers tearing from the stem bringing with them a thin layer of the quill that binds them.(*)
2. Prior to burning the shoulder feathers hold them down flat on a good cutting surface; using a new single edge razor blade or exacto knife, split the quill in half. After you have split the quills then proceed with burning and dying. This method is time consuming (to some extent) but you will get quick at doing it. I only do about 6 feathers at a time, this produces 12 usable, thin strips of hackle for 12 flies.
(*) While this technique does provide a very thin strip of hackle, that strip tends to be very curly and somewhat difficult to work with. These are also quite fragile both during the tying and when the fly is used for fishing. I go with the split quills although it can be tedious.