Hi, I think the best way to approach this is to break it into 3 sections; why this subject; why you; and are you interesting and unique. So, I'd definitely expand on what you've got - I'd probably aim to write 1 - 2 pages of properly spaced A4. The most important thing to remember is that this may be your only opportunity to sell yourself to the admissions tutor.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.