I could be wrong, but in my case my bones in my shoulders won't and haven't shrunk, but my posture shifted giving the appearance of a softer less prominent shoulder. Depending on the musculature you had before HRT, the pectoral muscles, biceps, triceps, trapezius and latissimus dorsi anchor the shoulder forward the more they are developed. Without as much upper body muscle mass, it's easier to keep your shoulders back in a relaxed feminine posture.
Learning to do this doesn't necessarily require HRT. Girls learn to do this, chest forward and shoulders back, as part of portraying confidence, as well as a means of prominently displaying their chest somewhat subtly. It's also seen as less aggressive/defensive compared to hunching them up or forward. It really is a matter of how you hold yourself, as irritating as it sounds. And it will appear to lengthen your neck if you aren't pulling it forward into a hunched position.
Hugs,
- Katie
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