There's no proof of "less breast devellopment" at 30 or over or even after that,
there's only the possibility that it will be significantly slower as HGH levels go down
say 10 years instead of 6, but that again is highly variable.
Some families have high variability in breast sizes, some don't.
So, if in your family its A to D, well its possible that you'll get A or D.
Some people have a high sensitivity to estrogen and even low levels
will give good devellopment while others need to pump themselves
to unsustainable pregnancy type levels to get a decent size.
Also, fat levels are important.
More athletic, fit women have less breasts in general.
I went from 36 almost C to 34B as a lost more than 20 pounds.
If your suffering from metabolic syndrome, too much weight in the middle,
this is a combination of too low E levels and too much fat and
processed sugar like glucose and not enough exercise.
This impacts where fats are stored, more are stored away from the breasts
and buttocks in this case.