Just thin out the tail by cutting some of the individual strands after you tie them on the jig. Synthetic fibers don't taper like natural hairs or feathers, so it doesn't matter if you cut them.

Those scissors with the comb teeth are called thinning shears. Pinking shears are the scissors with the zigzag blade. Hair stylists and dog groomers use thinning shears to take bulk out of clumps of hair without leaving a visible line of cut hairs. You can buy them pretty much anywhere beauty or dog grooming supplies are sold.