But you are likely to be asked about bigger methodological issues under-pinning the research choices you make. Since you are going to be asked about them in person, in your prospectus you ought to demonstrate that you are aware of the questions and what your answers are likely to be. For textual analyses you may well be asked about intepretive or language skills and how far advanced you are in them. As you can see - most of these questions address the issues of what are YOU going to do and how are YOU going to do it. This is a fairly self-evident category, but maybe the reasoning behind this requirement will help clarify why it is important.
Dissertations are quite bulky things.
In the past people have come up with proposals to write a dissertation that would essentially amount to a research note. It could well have formed part of a dissertation, but not necessarily the whole thing. Making you write down a chapter outline should force you to think of how you are going to fill 100-200 pages. The 10-15 page minimum should not be taken as an overall maximum though you would very rarely need to exceed 25 pages. You are free to include supporting material as an appendix.