My mbuna haven't grown as fast as I expected either. I've had them 14 months now. I'm not saying they haven't grown at all, just not as much as expected. I'm terrible at estimating sizes, but here goes:
Acei range from a 1.75" runt to a 4.5" dominant male, while the rest are in the 2.5" to 3" range.
Yellow labs range from a pair of 1.25" (born in my tank) to a pair of 3" females (but they were almost 3" when I got them). The rest are in the 1.75" to 2.25" range.
Maingano are all about 2.75" except one runt who is only 1.75".
Do these sizes sound small to you, or do they sound normal? I do multiple water changes per week. My nitrates are never above 10. I feed once per day (in the evening).