Hi marras
Other things being equal (eg weight etc) your breasts would be likely to shrink after menopause in the absence of oestrogen, and in your case as you haven't had HRT.
To me what is more surprising is not the decrease in the left one but why the right one hasn't decreased yet!
Before I went onto HRT and while I was probably in late peri-menopause - mine definitely shrunk and what was there had less substance to them - they were more floppy! Since I started HRT they have grown - and I would say maybe a little bigger than they were but not much, and definitely more substantial.
However weight changes also affect them - whenever I have lost weight in my life, with or without HRT, my breast size has decreased.
Glad to hear your tests were all OK and maybe the right one will follow suit!
Hurdity x