Nothing says that you have to use all 5300 characters. Your personal statement can be any length as long as you can clearly convey why you want to be a doctor. I've read some brilliant essays that are no longer than half a page and some total duds that manage to use up all the words counts to describe what the applicant did but didn't explain why he wanted to be a doctor. The longer essays more often than not read like glorified resumes (i.e. "From January 2006 to May 2007, I worked at lab blah blah and published 5 journals.... Then from May 2007 to September 2008 I worked at Memorial Hospital doing blah blah blah. Afterwards, I worked at..." and so on for five paragraphs) - these types of essays usually make bad personal statements.
If the reason why your personal statement is short is because you only spend a day or two writing it, then of course, I recommend that you find some substance to add to the essay. On the other hand, if you've spent 2 months writing and editing the essay and cut it down to about 3000 characters from say 7000 characters, then I'm going to trust that you did a brilliant job of editing and parring down the number of words to be succinct. If you have any doubts about your essay you can also send it to one of the reviewers and let them give you their opinion as to any changes that you may want to make.