Research proposal is usually 1-3 pages, and it contains the topic of your proposal, main objectives, introduction such as general info on were your topic came from, a brief review of research that was done on this topic and lack of research if there is one, and your ideas on how your research will fill this gap. Also it will include research questions you are going to ask, methodology and so on. SOP is a bit different because you talk there about your general career and research interests, what research experience you have, and that kind of things.