You will keep getting more and more ideas as you write the essay. All the ideas that you’ve presented in the article might not be included in the outline or the introduction. This means that you have a broad scope in expanding your opening, resulting in making the essay longer.
It also might happen that you’ve not mentioned the key points that were presented in the introduction. So review the introduction and write the essay according to that.
In the end, your essay should end in such a way that it leaves the reader with a clear idea or a solution. So write the conclusion to give an image or a resolution regarding the points or argument presented in the essay.
If you haven’t written the introduction, conclusion, or essay concerning each other, then go and edit it. All of this will automatically solve the question of how to lengthen an essay?