Consider this too...Sometimes the scar will decrease in size over time by itself. This is often true if you have been keeping it clean, not interfering with it's natural healing process. I'm not sure I would want to interfere with this process, it can wind up infecting or aggravating it, and making matters far worse. There are some chemicals on the market that actually inflame the injury, and will then result in a larger scar tissue size.
This too may depend on your horses particular ability to handle whatever remedy you may decide to use. Whenever practical, I allow the natural defense systems to handle what they were designed to do. Just a little assistance with keeping the area free from dirt, further injury, etc. is usually all that is needed.
Either way, good luck -Lw