If I play with 1 of the any of the 3 romans houses,like for example the House of Julii (red) and I have 10 cities cunquered, I can see in time that the other 2 romans houses, Brutii (green) and Scipii(blue) even if they have only 4 cities conquered,they have way much more army than me. And they do pretty well with income and constructing buildings in the same time.

What is the secret of them?

If you stop them to expand they will grow an huge army compared with the numebered of cities they have.

P.S. I play on the hardest mode.