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    <p>If you follow the <a href="http://en.wikipedia.org/wiki/Single_responsibility_principle" rel="noreferrer">single responsibility principle</a> (and based on the tone of your question, I think you do follow it), then the answer is clear:</p> <ol> <li>A class is too big when it does more than one thing; and</li> <li>A class is too small when it fails to fulfill its purpose.</li> </ol> <p>That's very broad and indeed subjective -- hence the struggle with your colleague. On your side, you can argue:</p> <ul> <li>There's absolutely no problem in creating additional classes -- It's a non-issue, compile-wise and runtime-wise.</li> <li>The <em>current implementation</em> of the service may suggest that these classes "belong" to it, but that may change.</li> <li>You can test each functionality separately.</li> <li>You can apply dependency injection.</li> <li>You ease the cognitive load of understanding the inner working of the service, because its code is smaller and better organized.</li> </ul> <p>Furthermore, I think your boss has a misguided understanding of <em>cohesion</em>. Think of it as <strong>focus</strong>: the narrower the focus of your program, the higher the cohesion. If the code on your satellite classes is merged within the service class, the latter becomes less focused (less cohesive). It's generally accepted that higher cohesion is preferred over lower cohesion. Try to enlighten his/her view about it.</p>
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    1. POWhen is a class too big or too small?
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    1. COThere is a problem with creating additional classes. In some environments extra classes mean more tables which means more queries which can result in slower performance. In web MVC environments this is the most evident. Furthermore, if no other classes use these "satellite" classes it implies that the satellite classes actually are part of that singular responsibility that the original class should be handling. More code does not mean less cohesion. You should also consider that it should be easy for another developer to create a single object and call a function without worrying about phases.
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    2. CO@Parris, database access performance is another topic which doesn't seem to apply in the specific case of the OP. About the scope of the singular responsibility, consider the *reductio ad absurdum* explanation in the answer by @David Thornley. And about developer convenience, it depends entirely on the public interfaces provided by the service -- you can have both high cohesion and ease of instantiation.
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    3. COI understand the reductio ad absurdum principal and yes division of code is absolutely necessary. In this case however I don't think it makes any sense. First of all, speaking from an OOP perspective, a class should mimic some sort of real world object. What is a ParseXML, MergeXML or a XMLService class. Parse and merge sound like function names not like classes. I suppose Parse and Merge can follow the adapter pattern in some sense. The structure described does not seem elegant :/. That being said, perhaps its the best solution in an OOP only paradigm.
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