It's tough to accurately answer your question without all the facts, especially knowing the genetics of the fish in question.
Some black diamond hybrids are staying smaller in the later generations (f4 and beyond.) I personally have some that have peaked in the 10-12" range. Males especially. A very good friend of mine has a BD hybrid breeding male that's app 8" disk and many years old. Over time, line breeding will cause the next generations to be smaller and smaller. This is why we are now seeing some of the hybrid and albino pearl lines staying on the smaller side (under 14") even females. The first generations were quite large (over 16" males and females)
Boesemani do get huge... That being said, breeding it to a hybrid that let's say for example is less than half it's size, does not guarantee the offspring will all still get huge. Some could stay smaller... It's fairly random. Need to grow them out. I hope this provides some clarification for you.