Reread your paper and look for any argument that would benefit from additional support. It is important to support your argument thoroughly with research and analysis. Discuss the strengths of your argument and explain why existing research opposing your argument is not valid.
Read your paper with an opposing viewpoint -- as your own devil's advocate, so to speak -- and think of any argument that someone could form against the argument you make in your paper. This will give you something additional to write about and lengthen your paper.
Ask peers or family members to read your paper and provide feedback on how to improve it. Specifically ask if something is missing or if something is unclear. Write about anything missing, or add to your paper to clarify anything that needs clarity.
Write a literature review or add to your literature review. A literature review discusses existing research on your topic, preferably giving an overview of research that supports and challenges your argument or topic. A literature review can strengthen your paper while adding length.
Add graphs or charts to provide a visual for some of the data or statistics used in the paper. While adding length to your paper, it also makes the paper more visually appealing and demonstrates your ability to incorporate appropriate graphics.
Ask a professor or faculty member familiar with the senior thesis requirements in your department to review your paper and suggest ways to add to the length. Professors are highly knowledgeable on many topics and can provide valuable suggestions. They will likely identify something you missed and provide you something more to write about. If they don't have suggestions, they may be able to direct you to someone who can provide suggestions.
Consult with other seniors writing theses on similar topics. Ask them for suggestions. Your peers can be excellent resources, particularly when they are working on similar projects and have knowledge on your topic.
Explain to your professor or adviser who will grade the senior thesis that your paper will not improve with length. If you have exhausted all other methods and you feel your paper is perfect the way it is, perhaps you are such a concise and thorough writer that it really doesn't need to be longer. Your professor can either direct you otherwise or decide to accept the paper's length.