Agree with Cherie, muscle mass is mostly genetic (though lack of good care/feed can cause a horse to remain smaller than they would have under ideal conditions), all that work will do is give you tone and definition.
For example, this mare looked like this when she came to me as an unhandled 3 year old. A little pudgy and soft.
And this was her nearing the end of her training. Many people would look at her and say "OMG, she's so skinny", but she wasn't skinny at all, she was just extremely fit at the time. She could easily lope 5+ miles without even breathing hard. She didn't get bigger, she just got stronger and fitter.
Here, you can really see the definition in her shoulder, she looks like a racehorse in that respect