Building encapsulation boundaries is a crucial skill in software architecture.
George H. Fairbanks in Just Enough Architecture
Balance the number and relative size of top-level components in your code.
Do this by organizing source code in a way that the number of components is close to 9 (i.e., between 6 and 12) and that the components are of approximately equal size.
This improves maintainability because balanced components ease locating code and allow for isolated maintenance.
A well-balanced software architecture is one with not too many and not too few components, with sizes that are approximately equal. The architecture then has a good component balance.
An example of component imbalance would be having a few very large components that contain a disproportionate amount of system logic and many small ones that dwindle in comparison.
Figure 8-1 gives an impression of component balance and what the ideal situation would be. The least desirable situation is on the top left because changes cannot be made in an isolated component. The ideal situation is shown last, the one with nine components in which it is most likely that maintenance can be isolated to one or two components that have a limited scope. The second situation (top right) suffers from a skewed distribution of volume over components. When one component is extraordinarily large, the architecture becomes monolithic; it becomes hard to navigate the ...