User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7153535-20130628164327/@comment-3146930-20130629002809

They aren't called the Big Three because their children are powerful, their children are powerful because they are the Big Three. Grover explains in The Lightning Thief that when they talk about the Big Three, they mean the three sons of Kronos as their territory encompasses most of the world (Zeus has the sky, Poseidon the Sea, and Hades the Underworld). Big Three is more of an honorary title, just a simpler way to refer to the sons of Kronos. It would explain why the Romans don't call them that.

The reason they stopped having kids was partly because their children were having too much of an impact on the world. The main reason however was that the Oracle told a prophecy saying that one of their children could possibly destroy Olympus when they turn 16.

Their Roman forms are more serious, so they may just not take having kids lightly. I mean Minerva is a complete virgin, not even having kids, unlike Athena. Jupiter is much more serious. We don't even know what Neptune is like as he hasn't appeared yet, but if the Romans feared him, he may be a scarier version of Poseidon. Their Greek forms are more free spirited with less rules, so of course they would have more children.