well I've gotten interview invites to all the schools I've applied to that have invited anyone yet. My PS was 1.75 pages.
IMO it all depends on how well you flow. Granted I'm not a PD or an application reviewer, just a dude who loves to write.
The first problem is that many medical students choose their specialty rather late in the game. Many if not most don't know what they want to do coming into third year. If you've only decided on your specialty less than a year ago, you don't have a very long story. Now, your enthusiasm might be such that you have a great amount of detail beyond an ortho prospect saying "I love the gym and I want to cut", but detail is not the same thing as plot. Detail is tedious. Detail slows pace. Detail in and of itself is not your friend.
The next issue is that most med students are not writers, in my experience. At worst, you come off stilted. Best to say your piece and leave it at that. At best, it reads much like a research paper or a news article. Neither pace nor flow.
Now, even if you can flow, that doesn't mean you have pace. 500 words can "read" slow or fast. It's a matter of energy and enthusiasm conveyed through the written word. You have to ratchet up the speed with which the audience reads your piece. It's a very real phenomenon. I've read a few essays where the student was able to flow, but not able to produce the energy to get a piece read quickly.
That is by no means a bad thing, and great authorship is great authorship, but you have to know your audience. In this case a hurried and overburdened PD.
If you've got a story, if you can flow, and if you can imbue your piece with energy, you can easily double the length of an average PS. If you can't, you'll only be hurting yourself.
Now, I could also be absolutely 100% full of crap, and it's entirely possible that they didn't read through my entire personal statement and invited me solely on the strength of my application and LORs. All I know is that there was no way to tell the story shorter than I did, and I'm happy with the result. *shrug*