Why do some hot water heaters need thermal expansion tanks and others do not?

I believe that the larger the hot water tank, the greater is the need for an expansion tank. Water expands as it is heated. This is why all hydronic heating systems require an expansion tank. A water heater would need expansion tank when a backflow preventer or check valve is installed in domestic cold water line directly after water meter. Some areas require backflow prevention, but not all. Have to check with local Plumbing authority about that.