Nipples being less puffy is part of the maturing of the breast; the puffy nipples stage is supposed to gradually change to the nipples becoming part of the contour of the breast again. (Some women, cis and trans, have puffy nipples forever, but that's less common.) Also, don't forget that the bigger they are, the more they will seem to visibly contract when hard/erect - tiny nipples don't change much, because there isn't much there to tighten up. So you should expect them to look distinctly bigger or smaller depending on temperature, stimulation, etc. (Pre-HRT, I could barely tell if my nipples were hard; now, they shrink by about 2/3 when they're erect [though they do project more, obviously].) It sounds like what's happening is that you're actually advancing into the next stage of development, which is good.