Not all temples are created equal. In my Julii campaign, I built up Ceres temples until I got the last city upgrade, destroyed them and replaced them with Jupiter temples. I also kept at least 4 units of peasants in all of my cities. Carthage had 100% loyalty to me when I finished and had around 32,000 people but still had 1% growth. The problem is a combination of grain imports and high base population growth.