When managing complexity of the system, it’s important to optimize both complexities - local and global.
When optimizing only global complexity, it leads to all functionality implemented in a one monolithic service. It may be okay, but also may lead to the big ball of mud in local complexity.
When optimizing only local complexity and neglecting the system’s global complexity it may lead to a distributed big ball of mud.
Deep module concept can be applied to address this. A deep module reduces global complexity, and the Shallow module increases it.