yes the Brutii are the richest of the Romans, still only because they have quick expansion possibilitys to greech and Asia-Minor.

However on that pic. they do not seem to hold really rich provinces , only thapsus, and Libyum ( sp ? city on Sicily ) and thermon,and their italian city's- at least that is what I can make out from the screenshot.

So the only thing what they could have done , is cheat , or just built economic structures only - and / or just have very very small army's.