I'm sure you know this, but theoretically, the more that's going on visually, the higher the bitrate of the video is. And therefore the more data is being written to the card in a shorter amount of time.
For instance if you're recording something in a very dark place or without much going on visually, even on the highest quality setting, you'll actually get a lower bitrate and smaller overall filesize because of it. I can't speak for this camera, but I've seen it myself with others, and I'm guessing this camera uses VBR recording as well.