How can essay word counts be modified

Any student surely is familiarized with the feeling: you feel you have written the perfect essay, with quotations, references, plenty of examples and a full conclusion with many new ideas. But somehow, you check the essay word count in your textbook processor and you find out you have written only a small fraction of what you were supposed to write. Do you have to start all over? How important is the amount of words, really? If you have an important idea to express, does it matter if you say it with not too many words?
The opposite situation is also possible. You have spent thousands and thousands of hours in front of the computer, writing your essay. But suddenly you discover you wrote more than you were asked to. So, it doesn’t matter if the essay is great, it is just too long and you have to erase some parts of it. After all, too long essay word counts can be as bad as short ones. Or can’t they?