Tits size is a feature. There is a chance tits will be bigger with new generation of females.
For example, Human + Goblin , there is a chance to get Goblin Female, and she can have bigger tits, you just have to wait. But there is also very small chance to get human female as well, including bigger tits.
If you have Tits v2 by Krongorka, then your max size tits are EXTRA.
Before brothel, you will have very small chance to have human/elves etc with better genes over time, but it's not impossible.
Maybe I should update tips on my post about Complex Breeding.