It's a sad fact that muscle never goes away, unless you starve to the point where your body starts breaking down protein, and then it attacks the brain first, which results in brain damage before other muscles actually start decreasing in mass.
However, muscle cells do change direction under the influence of estrogen or testosterone. I'm sure this is an oversimplified explanation, but basically, the muscle cells are elongated, and they turn 90 degrees and become shorter and taller under the influence of testosterone. The result is greater strength, and more bulging appearance. So for most MTF, they evetually revert to the original position and become longer and thinner under the influence of estrogen. The result is less strength and less bulging appearance. I don't know if that can happen for someone with complete androgen insensitivity, since that person's muscles will theoretically be in the female position already. However, even if that is the case, it might still happen to some extent for someone with partial androgen insensitivity. I really don't know. But I've heard that the process can take a long time. If I recall correctly, I've heard that it can take up to 7 years with no testosterone exposure and only estrogen exposure.
However, the female face, especially the lower part, tends to get more of a layer of fat under the influence of estrogen, which can quickly decrease the appearance of the jaw muscle. Also, it might be possible that the bone under the muscle is pushing the muscle out and making it look bigger, and that a trip to one of the FFS surgeons could reduce the bone to some degree. But I would suggest waiting to complete transition and then waiting some more to let the years pass and revisit it. My experience was that transition can stall until surgery is completed, and then the hormones can be more effective. But maybe that's not applicable to a person with androgen insensitivity.
