I think it really depends on how muscular you were prior to MTF HRT. From my experience - the gist of HRT slams down the T level, but really diet and exercise influence the muscle development/regression. Yeah, with decreasing T some muscle mass will decrease, but the underlying muscular base will remain. In terms of the pecs/breast issue - I see it as a similar scenario to off-seaon/out of shape bodybuilders who use anabolic steroids and the proverbial "bitch tits" (pardon the expression). The pec muscles will be less defined, and breast glandular tissue will be noticeable (esp. if you know what to look for...as did all the male bodybuilders when comparing their chest to anothers...always looking to see who was juicing). Again - depends in large part to how built you are/were, and how long the muscular base has been there.
I was a former bodybuilder (as opposed to someone who simply worked out or used weights). Even though I've not lifted a single weight in over 5 years - I still have native mass and ridiculous muscle memory - any prolonged weight bearing physical activity (i.e. when I spent one month moving out of my old apt) will trigger muscle growth again, and mitigate the appearance of breast development. All this being said, however, just remember that the pectoralis group sits below the breast glandular tissue and adipose, and they therefore operate and grow/regress independently. It's just that if you have a well-developed pectoralis base, the developing/developed breast tissue may seem abit dwarfed (compared to another chest without well defined pecs). I can feel where my breast tissue is, and I can feel where my pec muscles are. The breast tissue is not adversely affected by my pecs...they're just not as obvious as in less muscular girls.
