Muscle Building Supplements?

So I've decided to keep Whisper, again. When it comes down to it, I just really don't want to part with her. I can teach her to run barrels, even if she's not great at it, we can still have fun until I'm ready to move up. Her size is still bothering me a little though. She's only 3 years old, and I'm sure she will still grow a little more. But at 14.1hh and as petite as she is, I still feel huge on her.

So my question is, would a muscle building supplement help her to fill out, or is that not a good idea with such a young horse who still has growing to do? I don't know how these things work, and I want to make it as comfortable as possible for both of us. She's almost at her max weight carrying capability with just me, going by the 20% rule.

Any thoughts or suggestions?

IMO, natural muscle building is the way to go, especially with young ones. At her age she really should just now be getting into the tougher riding, although it sounds to me as if she's already been doing quite a bit of work under saddle. I would focus on building up her flexation, yeilding, and work on lead changes, and do lots of uphill walking/trotting if possible. All of those things will help her develope the muscles she needs to perform well, as well as the flexability and versality needed for western events. At three, it's perfectly normal for her to look rather scrawny, also. I don't find many horses who begin filling out until their fourth year.

Chin up, she's young! She might just have that barrel horse heart in there, somewhere. Let her have fun and develope in her own timing. That's the way to get the soundest, happiest horse.

She will still bulk up as she gets older{5-6yrs old},at this point they don't grow so much up,but out.She is only going to grow according to her genetic make-up,she either has the natural bulk or not.