I wouldn't worry about word limitations as much as how well-written and sincere your statements are. If it says no more than 500 words, and you've written an extra 100 that are truly necessary for your essay to support why you want to go to that program, I'd leave it. If the extra words don't add anything, edit them out. My statements are 940-960 each. I have friends who've exceeded 1000 words and gotten into their top choice schools. I think that if you can be concise yet compelling you are fine, but a longer essay that is also compelling may suit some schools.