It's listed as a 13 MB app on the App store. It could appear smaller because it hasn't launched or decompressed. But like I mentioned before, some have said core apps get updated via OS updates. If an app is downloadable then it could be updated on an as needed basis.
And I've certainly had it take up way more when various podcasts were cached by it (deleting the app and installing it again would clear it out, but can't be done now that it's a stock app basically).