Do you mean something like "This is really short. I'm probably skipping a bunch of steps in my reasoning and need to spell it out more explicitly" or do you mean something else?

Kindof something else, though I think you get the idea.

For any paper, you can always explain more steps in your reasoning, define your terms better, or bring out more of your assumptions. One of the good heuristics for determining how much you need to do this is to consider your audience. However, when writing for a class for your professor, you don't really have this luxury (if you're writing on something noncontroversial, your argument could be written as a conclusion followed by 'you already know the rest'). Since part of the purpose of writing the paper for the class is to demonstrate whether you understand the material, you need to explain to the professor things he already knows. Paper length is a good rough heuristic to let you know just how much of that you need to do in the course of your argument.