This has been known for at least 30 years.
The change this century has been a calorie change. People in the 20s and thirties ate much less than they so today. Parenting philosophy was about strict standardized mealtimes. Less calorie and vitamin intake led to smaller people.most of us are much larger than out great grands.
Menarch is precipitated by weight and fat content of the female. Rule of thumb has been approximately 103 pounds. I suspect it is less recently. Signs of puberty such as breast swelling and pubic and axillary hair is beginning as early as ages 6 or 7 and the threshold for precocious puberty had dropped to ages 4 or 5.
Fats are the precursors of hormones.
I think that you are primarily correct.
I suspect the the insane number of vaccines now given to children could come into play into addition to that to a lessor degree.