We use domain decomposition because we want to exploit the power of more than one processor. As a consequence, the right question to pose is: "How do we need to partition the domain so that we get the maximal speedup by using as many processors as subdomains?"
The answer to that question is "subdomains need to be chosen so that the work necessary on every subdomain is equal among all processors", because if it isn't, some processors will sit idle while others are still working. In practice, that typically means that for standard finite element discretizations, the (global) mesh needs to be partitioned in such a way that every processor has (roughly) the same number of cells. (I will note that this isn't true if you have, for example, different polynomial degrees on different cells, or are doing something different/additional on some cells than on others.)