Your past research experience is certainly a big part of it, but it's definitely not the place to write all about your research findings and write a research proposal for what you plan to do in graduate school. Write about what you accomplished, but use it to highlight what you learned by participating in research and how it has better prepared you for graduate school. Nobody wants to read what you published in the experimental section of your paper, but they do want to know in what capacity you were engaged in research and how much you were involved in the process. You are right that they most likely will not look at your paper, because that doesn't tell them anything they particularly want to know. Even if you were first author on that paper, your contributions are still ambiguous. Make your accomplishments clear in your SOP, and again, focus mostly on how the experience affected you and what you learned, not necessarily the main results of your publication. Mention what you are interested in doing in the future, but don't write a research proposal. Basically this essay is your chance to tell the admissions committee about who you are as a person, and highlight the reasons why you think you would make an excellent graduate student and scientist.