The burundi are the largest. I would say one is at least 9. The other is close to that. I try to short the measurements incase I am off, but when I measured the male from the back of the tank last week he was what seem like 10. My wife says bigger.
The Mpuimbwe are smaller, but in with 10 inchers and have no problems. I have one Mpimbwe who is like two inches... but he is in another tank growing out. The other two are about 5-6 inches. Fish are priced to move and based on what they are about really worth. Mpimbwe are more expensive than regular burundi, so the size difference is taken into the price of the fish.