It has nothing to do with unit size, I usually play large, but have also played on normal, it's not that much difference.
The Sweboz have the advantage of being surrounded by rebel provinces and can expand in all directions without much resistance, the Romans have a lot of money because they have rich provinces in Italy and expand around the mediterranean which means a lot of money through sea trade. The gauls are busy fighting each other plus the Romans and the Sweboz and do expand rather slowly. So they can't make that much money.