Well with a bit of digging I did happen upon this video:
In it there is conversation and examples of the SIMS4 "genetics"
Especially around the 2:15 mark where they show a quick "twin" of a male character without any genetic contribution from a female character - this is an actual XX from a single XY character.
Why this is telling is twofold:
1) earlier they show the male's mother who has what appears to be a decent sized chest for a SIM and all genetic combinations with actual women appear to attempt to retain that size when dealing with a family line - and because she is larger she tends to average out even the contributions from flatter combinations when making a female offspring in subsequent generations.
2) as soon as alternate methods are employed, a clone, a twin sibling, a randomized non-family mix it appears that the game dumps the mother line and does one of two things.
a. if there is no mother (clone, twin sibling, etc.) the game appears to use just the male - therefor any woman based on them is rather flat.
b. if there is a female reference (sister, random mate, etc.) it appears to use just the female's shape to make female characters.
It just so happens that the base male is slightly stocky so even the female characters without a female reference do have a slight chest and that may be one of those built-in limiters of the game.
In other words, in the SIMS, if you are female you will have a minimum bustline.
Near the end they do show that particular aspects can be locked while letting the randomizer affect the remaining. It is telling to see how the categories are grouped. From that it is obvious that bust size was very low on the programer's awareness in terms of genetic consistency.
Conclusion:
SIMS4 essentially uses the "mother" if available to size any offspring.
The size will not necessarily be an average but it will never exceed the mother. This appears very set when generating family lines with the busts looking to be locked at the mother's size.
As soon as there is no mother in the calculation it will use the male which enviably results in a flatter female.
I find it odd that the game is so advanced when combining two genders and becomes so myopic when simply modifying from just the male line. It obviously knows that a male had a busty mother but seems to dump that knowledge when creating unconventional variants (no XX contribution).
This points to just one thing -
The female characters have a "bust" category of traits not maintained with male character's stats.
As long as there is a female in the mix that trait and following generations have a reference, such as with the family line. Drop that female reference size, like with a clone, and the game will either make a rather flat woman (minimum bust) or randomize a size irregardless of the paternal line's contributions. Both cool and disappointing at the same time.